On Joomla if you have the Securitycheck Pro extension, it can block the template save process. You can add an exception in it for AcyMailing.
If you have modified your .htaccess file and some form validations don't work anymore, you should modify your rules to replace [L] flags by [P] flags.
If you have Jyaml as template, enable/disable the option for "empty" module
If you have the System plugin Responsive scrolling tables, disable it.