Title: Plugins problem in multisite installation Last modified: February 10, 2026 --- # Plugins problem in multisite installation * Resolved [Quique5](https://wordpress.org/support/users/quique5/) * (@quique5) * [3 months ago](https://wordpress.org/support/topic/plugins-problem-in-multisite-installation/) * The latest version where the plugin works correctly is **6.14.2**. Versions released after that fail in a multisite installation. * When reverting to version **6.14.2**, the plugin works correctly again. * The issue occurs after updating to **6.15.15**, as well as some earlier versions. * here some logs: * [Tue Feb 10 12:30:21.325129 2026] [cgid:error] [xx] [client xx] AH01215: stderr from /var/www/cgi-bin/php74.fcgi: PHP Fatal error: Cannot redeclare check_upload_size()( previously declared in /home/www/multisite/htdocs/wp-admin/includes/ms.php:20) in /home/www/multisite/htdocs/wp-admin/includes/ms.php on line 20, referer: [https://www.fuenxxx.es/login/](https://www.fuenxxx.es/login/) * [Tue Feb 10 12:30:32.253079 2026] [cgid:error] [xx] [client xx] AH01215: stderr from /var/www/cgi-bin/php74.fcgi: PHP Fatal error: Cannot redeclare check_upload_size()( previously declared in /home/www/multisite/htdocs/wp-admin/includes/ms.php:20) in /home/www/multisite/htdocs/wp-admin/includes/ms.php on line 20, referer: [https://www.alaxxx.es/login/](https://www.alaxxx.es/login/) Viewing 10 replies - 1 through 10 (of 10 total) * Plugin Support [tristan083](https://wordpress.org/support/users/tristan083/) * (@tristan083) * [3 months ago](https://wordpress.org/support/topic/plugins-problem-in-multisite-installation/#post-18817358) * Hi [@quique5](https://wordpress.org/support/users/quique5/) , * Thank you for reaching out. * Unfortunately, I have not been able to reproduce this issue in my multisite testing environment running only The Events Calendar plugins and the twenty-twenty-one theme. * For us to understand and learn more about your setup, could you provide the version of the following? 1. PHP 2. WordPress 3. The Events Calendar 4. Events Calendar Pro (if using) 5. Event Tickets (if using) 6. Event Tickets Plus (if using) 7. Theme and Version * We would also appreciate it if you could share the full stack trace of the error that you got in the debug log. * Thread Starter [Quique5](https://wordpress.org/support/users/quique5/) * (@quique5) * [3 months ago](https://wordpress.org/support/topic/plugins-problem-in-multisite-installation/#post-18817504) * Thank you for your response, * PHP 7.4.33 WordPress 6.7.1The Events Calendar 6.14.2 (lastest working) 6.15 and beyond failsEvents Calendar Pro (if using) NOEvent Tickets (if using) NOEvent Tickets Plus (if using) NOTheme and Version Divi 4.27.5 * The only errors I have are them, because I quickly downgrade to 6.14.2 (it happened some months ago). That time I edited ms.php and when I commented the line with‘ check_upload_size()’ the error chage to other function * Plugin Support [tristan083](https://wordpress.org/support/users/tristan083/) * (@tristan083) * [3 months ago](https://wordpress.org/support/topic/plugins-problem-in-multisite-installation/#post-18822224) * Hi [@quique5](https://wordpress.org/support/users/quique5/) , * Thank you for your message and the information. * Unfortunately, I have not been able to reproduce this issue in my testing environment( see [https://somup.com/cOnojvWZYD](https://somup.com/cOnojvWZYD)). I suspect there’s a conflict that’s causing the issue here. It would be good to test for conflicts. Could you follow the steps below? 1. Deactivate all other plugins except for The Events Calendar plugin. 2. Switching to the [Default theme](https://codex.wordpress.org/WordPress_Default_Themes). 3. Update The Events Calendar plugin from 6.14.2 to 6.15.16. * Please read the [Testing for Conflicts](https://theeventscalendar.com/knowledgebase/k/testing-for-conflicts/) article before doing anything.  To be safe, we’d recommend conducting tests and updates on a [Staging Server](https://theeventscalendar.com/knowledgebase/k/creating-and-using-a-wordpress-staging-site/). Also, please keep a working backup of your website. * Plugin Support [Darian](https://wordpress.org/support/users/d0153/) * (@d0153) * [2 months, 4 weeks ago](https://wordpress.org/support/topic/plugins-problem-in-multisite-installation/#post-18824433) * Hi there, * I just wanted to follow up and check whether you’ve had a chance to review the recommendation above. * Please feel free to let us know how it went, or if you have any additional questions or concerns. * Thread Starter [Quique5](https://wordpress.org/support/users/quique5/) * (@quique5) * [2 months, 3 weeks ago](https://wordpress.org/support/topic/plugins-problem-in-multisite-installation/#post-18827230) * Hi there, * I have tried with PHP 8.3 and the latest version of WordPress, and the error persists. I have also disabled some other plugins, but nothing worked. * I’m still investigating. * Plugin Support [Darian](https://wordpress.org/support/users/d0153/) * (@d0153) * [2 months, 3 weeks ago](https://wordpress.org/support/topic/plugins-problem-in-multisite-installation/#post-18827430) * Thanks for your response, [@quique5](https://wordpress.org/support/users/quique5/) * To help us rule out any plugin conflicts, please try activating only our plugins along with the **Twenty Twenty-One** theme and check if the issue still occurs. * If the issue is resolved, please reactivate your plugins and theme one at a time, testing after each step, to identify which one is causing the problem. Let me know how it goes. * Plugin Support [Darian](https://wordpress.org/support/users/d0153/) * (@d0153) * [2 months, 3 weeks ago](https://wordpress.org/support/topic/plugins-problem-in-multisite-installation/#post-18830853) * Hi [@quique5](https://wordpress.org/support/users/quique5/) * I just wanted to follow up and check whether you’ve had a chance to review and try the recommendations above. * Please feel free to let us know how it goes, or if you have any further questions or concerns—we’re happy to assist. * Thread Starter [Quique5](https://wordpress.org/support/users/quique5/) * (@quique5) * [2 months, 3 weeks ago](https://wordpress.org/support/topic/plugins-problem-in-multisite-installation/#post-18831000) * Hi [@d0153](https://wordpress.org/support/users/d0153/), * First of all, thank you for your time. * I have updated WordPress to the latest version, but that is all I have been able to reproduce so far. In two days, I will be able to set up a test environment to investigate the issue more thoroughly. I will also try deactivating some plugins. * I won’t be able to investigate further until then. * Plugin Support [Darian](https://wordpress.org/support/users/d0153/) * (@d0153) * [2 months, 3 weeks ago](https://wordpress.org/support/topic/plugins-problem-in-multisite-installation/#post-18832444) * Thanks for your response, [@quique5](https://wordpress.org/support/users/quique5/) * Please feel free to reach out anytime to let us know how it goes, or if you have any other questions or concerns. * Plugin Support [Darian](https://wordpress.org/support/users/d0153/) * (@d0153) * [2 months, 2 weeks ago](https://wordpress.org/support/topic/plugins-problem-in-multisite-installation/#post-18838009) * Hi@quique5 * This thread appears to be inactive, so I will proceed with closing it for now. If you have any additional questions or concerns, please feel free to reopen this thread or create a new one. Viewing 10 replies - 1 through 10 (of 10 total) You must be [logged in](https://login.wordpress.org/?redirect_to=https%3A%2F%2Fwordpress.org%2Fsupport%2Ftopic%2Fplugins-problem-in-multisite-installation%2F%3Foutput_format%3Dmd&locale=en_US) to reply to this topic. * ![](https://ps.w.org/the-events-calendar/assets/icon-256x256.gif?rev=2516440) * [The Events Calendar](https://wordpress.org/plugins/the-events-calendar/) * [Frequently Asked Questions](https://wordpress.org/plugins/the-events-calendar/#faq) * [Support Threads](https://wordpress.org/support/plugin/the-events-calendar/) * [Active Topics](https://wordpress.org/support/plugin/the-events-calendar/active/) * [Unresolved Topics](https://wordpress.org/support/plugin/the-events-calendar/unresolved/) * [Reviews](https://wordpress.org/support/plugin/the-events-calendar/reviews/) ## Tags * [Multisite error](https://wordpress.org/support/topic-tag/multisite-error/) * 10 replies * 3 participants * Last reply from: [Darian](https://wordpress.org/support/users/d0153/) * Last activity: [2 months, 2 weeks ago](https://wordpress.org/support/topic/plugins-problem-in-multisite-installation/#post-18838009) * Status: resolved