OK - this isn't particularly easy to fix. The reason mediawiki doesn't support underscores is because it is used as a delimiter for some urls where the username is embedded.
The simplest and most effective way for me to resolve this issue would be to change the internal username within the forum's database for those who are affected. I can do this without affecting the user's 'nickname'
So for example, Cyprian_K's login could be changed to 'cyprian-k' or 'cypriank' but will still appear as 'Cyprian_K' within the forum. The internal username can then be used to authenticate with the wiki and the forum without issue.
I have tried this for a dummy account and it works well.
Moving forward, I will disable the use of underscores in forum usernames for new accounts. For those who already have usernames containing underscores and would like me to make it wiki-compatible, please PM me and I will manage these on an individual basis.
STE: Desktopper case, IDE interface, UltraSatan (8GB + 512Mb) + HXC floppy emulator. Plus some STE's/STFM's