When user registers on my forum on http://www.zskajov.cz/forum2/ or http://www.zskajov.cz/forum/index.php using user name containing capitals they are unable to login with the same username. They have to enter the username in lower case. For example if zou register as "Username" you need to login as "username". Do you have any idea how to fix this?

Make sure to un-comment:

//$loginsCase=TRUE;

in setup_options.php (check the manual for more information).

I suppose it also could be related to some issue with the database. If you have allowed some specific (chezh for example) symbols in the username, database could handle them incorrectly. Anyway this issue appears not to be related to miniBB (as you could see here the logging routine works in any case)...