Hi,
When you have such error, the first thing to do is to activate the "error reporting" and "debug mode" settings of the Joomla configuration.
That way, it should display the call stack of the error (a list of files, functions and lines the code went through up until the error).
This alone should help identify where the problem comes from.
On our end, the only things which link with AcyMailing 5 are the plugin "hikashop" of the group "acymailing" and the plugin "acymailing" of the group "hikashop". So if you're using AcyMailing 6, you need to make sure that these plugins are turned off in the joomla plugins manager.