Concerning the Re-entry of key issues. Today I purchased v3.3 for windows and installed it as admin with antivirus turned off and rebooted. I also had the same issues. Upon a quick look at the registry, it appears that the regcode is copied to 3 locations, but one of the locations is not updating the regcode. This is a design flaw in the program and not a Windows thing. I'm running Windows 10 Pro 64-bit with all the updates - today is 1/12/2019.
Regedit keys go here:
HKEY_CURRENT_USER\Software\SereneScreen\MarineAqua rium3
HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\SereneScre en\MarineAquarium3
HKEY_USERS\S-1-5-21-1*********-16********-********6-**00\Software\SereneScreen\MarineAquarium3
...but the regcode is not getting written to the HLM (HKEY_LOCAL_MACHINE) section.
I copied it there from HCU and all is working fine now.
Hope this gets fixed on v3.4
Cheers!
-Galo