Well that was my concern too. I'm running R80.10 and thought maybe those commands are different now. That wasn't the case as I ended up figuring out the solution to my problem.
Every time I ran "fwm dbexport -f users.C" or "fwm dbexport -g -f groups.C" I would corrupt my database causing the "Failed to convert database" error. I then had to follow the steps laid out in sk112824 to rebuild the database. It appears the reason I was corrupting my database was because I had to use ./ in front of users.C and groups.C otherwise it was putting the files in the $FWDIR/conf/ directory. As soon as I specified an output directory the commands ran successfully and did not cause a database corruption.
EDIT: I also opened a ticket with Check Point and they referred me to sk110519 stating that fwm export isn't supported. I guess even though I can run the commands they just put out headers and no data.