I wanted to report that I ran into this same issue with our 3CX Professional system today. I needed to restart services for another purpose last night, and it reverted to the PBX Edition feature set and call capacity but didn't reflect this fact in the license dialog or in the dashboard, nor did it send me an alert email. The solution noted above worked, and the Professional license works again. I found out this had happened by getting 'License Limit is reached' emails this morning; I went to do a call report, and got the 'Feature not available in PBX Edition' message. Searching found this thread; this solution worked.
Version information: 15.5.1694.0, 32 simultaneous calls, Professional Edition, running on Windows Server 2008R2. I did get the call report done after the reactivation, and the limit was reached at the second incoming call (we ended up missing calls this morning). I would have liked to have gotten an email alert that it had reverted to PBX Edition, or some indication in the dashboard that this had happened so that I could have found this thread and fixed it last night before missing calls this morning.