Issue with synchronizing data

Custom Contact Forms is experiencing issues.
There is an issue with synchronizing data. Please try deactivating all other plugins and activating the TwentyFifteen theme. If this fixes the problem, you have a plugin or theme conflict. If it does not, please post in the support forums or Github.

i getting this popup box when i save the form.

Please advice

https://ww.wp.xz.cn/plugins/custom-contact-forms/