Hi,
in the version 6.7.x, our developers decided to rename AcyMailing 6 to AcyMailing. If you have older verison of AcyMailing 5 (version 5.10.11 or earlier), which is also named as AcyMailing, you will have the exact issue as you have
Solution for this is really simple.
Update version 5 as well
Regards,
Igor