When one of my teammates try to lock a file that is already locked by another developer, the prompt that says
the file(s) are locked always shows "?" instead of the username of the person that locked the file.

We tried the following steps to solve the problem:
Upgrade AnkhSVN and VisualSVN Server to their latest versions (AnkhSVN 2.4.11610) (VisualSVN 2.5.6)
Upgrade the repository format to Subversion 1.5 format
*The above steps didn't have an effect at all, so what we did later was to downgrade to AnkhSVN 2.3.11269

Even after downgrading, It still didn't have an effect - when people try to lock files that are already locked,
it still shows "Locked by: '?'".

P.S.
Our project is a windows forms project and we use Visual Studio 2010 SP1Rel
TortoiseSVN reports the username of the developer who locked the file correctly