I was getting the same error, while I was trying to use this plugin for serial numbers, which is very good for generating specific IDs. After disabling it, works well. But I need the plugin. Bad situation.
https://ww.wp.xz.cn/plugins/contact-form-7-serial-numbers/
I can confirm V 2.0 is working in the environment with changed /wp-admin/ URL to something else.
Thank you very much, Fernando, for the quick fix.
I would say, it is not random issue. In my case, it is caused by changed default admin /wp-admin/ URL to something else like eg. /administrator/.
Thank you for the update.
Can we know the reason why was plugin removed from ww.wp.xz.cn repo?
