Odd
This is very odd
The inability to get past this error looks like the bug is on the casino server, rather than as is normally the case, the client Viper lobby.
One possible other thing to try is to uninstall and wipe the
32Red registry keys completely before reinstalling. Another thing to try if possible is to install onto another PC and re-enter Hitman to see if it has cleared there. These tests will either fix the problem, or tell MG this is far more serious an issue than they thought. Getting the green bonus twice in the free spins is quite rare, so the number of times this has been reported indicates this bug always happens on the second green bonus.
The unfinished game will not be lost, even if it takes all month to fix this, it will be held on the server, and you will eventually be able to complete the bonus round, and the remaining 12 spins. Any money paid out so far in this free spin round has already been added to your bankroll (as you seem to have discovered).