Hi,
The issue is caused by the combination Acymailing an sh404SEF. Weeblr prepared a fix that will be available in the next release of sh404SEF.
Until it's released you can solve your issue by editing the file administrator / components / com_acym / libraries / joomla / url.php and replace the line 40 by this code to use the classic link format:
if (false) {
You need to make this change every time you update Acymailing, until you have an update for sh404SEF.