Sometimes it happens after the system update, just try click Deactivate and then re-register again.
If it’s a serial number issue, you may need to contact the software vendor or send a message to Dreamtonics.
(Translate with translator)
I tried deactivating but it didn’t work; however I did manage to fix it myself. What I did was I uninstalled Synthesizer V and deleted all the program files, then reinstalled everything. That allowed me to re-enter my serial numbers; everything works now.