Please turn site debug to on, so that you see where the error happens (PHP file and line number), as well as call stack, and maybe you also have some hidden notices and warnings before this error.
Since when or after what did you start to have this issue ?
Did you migrate the site from Joomla 3 ?
Or had 3rd party or old plugins that you removed without uninstalling ?
Are your extensions, incl. AcyMailing and AcyMailing CB integration up to date ?
Maybe this similar issue in this thread, also with AcyMailing, can also give you a hint on the solution:
www.joomlapolis.com/forum/professional-members-support/245520-cannot-edit-user-from-backend?start=12#332779