I get this error when trying to edit the acym module in BE.
AcyM v7.9.3, Joomla! 3.10.10, php 7.4
We can't solve this on the forum. Please raise a support ticket (https://www.acymailing.com/support/) so we can further assist you.
I have the same problem and was wondering if there is already a solution for this issue.
Please check if the plugin "AcyMailing - Joomla integration" is enabled. (Joomla->Extensions->Plugins)
It must stay enabled to ensure the other AcyMailing features to work properly