I have a problem with the acymailing extension and I have contact with their support team.
The answer is this:
We use a function from sh404SEF to generates URLs when inserting the article.
It seems they modified their code in one of their recent versions and calling this function on the back-end is creating an error.
You should contact their support telling them that calling their functionSh404sefHelperGeneral::getSefFromNonSef($nonSefUrl)from the back-end is creating an error on the line 635 of the file /components/com_sh404sef/sef_ext.php
The error is:
Call to undefined method Joomla\CMS\Application\AdministratorApplication::getParams()
Can you do something for that bug?