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.
If AcyMailing was already installed when you turned your WordPress website into a multisite network, you may need to re-run the AcyMailing installation scripts to set it up correctly for your network. To do this you can update AcyMailing, upload the latest installation package to replace the current one, or you can follow these 3 steps:
Disable AcyMailing (this won't remove its data)
Create an empty file wp-content / plugins / acymailing / update.php
Enable back AcyMailing
If this happens on a Joomla website, it could be caused by the system plugin "AntiCopy" when framing is Disallowed. Change its options or disable this plugin to fix the issue.