Guys, the reason for this problem is the fact that you probably migrated your site from one domain to another, which created a havoc in the Joomla update sites DB tables
Solution for this problem is actually quite simple.
- Go to Joomla Installation manager and under the Update sites find AcyMailing entry
- remove AcyMailing entry
- install manually an update or the same version that you already have
This procedure will properly add new entry to the Update sites Joomla DB tables and all future updates should work without any problems