That's impossible, since standalone installer is creating unique CD-key for you that should never give you the "invalid CD-key" message, so probably something went wrong during your installation. For help, join our Discord, and check the "tech-support" channel. CD-key changing in the registry has been recently discussed in there, that should help you.

I took some time and sat down to try and solve this. For those who might stumble upon this post:

I figured my previous various installations of BF2 might have messed up my registry and what not. So I uninstalled everything related to BF2 (base game, Special Forces addon, FH2, however not PR). I did this through the old "Programs and Features" control panel, not the new Win10 Program list (older programs don't show up there).

Then I fired up CCleaner and did a registry clean (it found several obsolete FH2 and BF2 entries).

Finally, I installed the FH2 Standalone version.

I'm happy to have FH2 working again, see you on the battlefield!