Hi Auge,

That could cause problems. The Forum script, the database and the database connections are handled in UTF-8. ISO-8859-1 is a (small) part of UTF-8, but with the first special chars (like umlauts) you'll have your first problem.

server_default_charset was used until version 2.0 RC 2 to properly convert date strings into UTF-8. This is necessary, because locale doesn't return UTF-8[1] (from 2.0 RC 3 on it is recognized automatically).
It has nothing to do with the charset used by the forum.

[1] e.g. de_DE.UTF-8 does but I'm not sure how widely spread it is yet.

Alex