VisualSVNServer problem authentication via web interface

we have installed VisualSVN Server on a local domain machine. We had used it for a long time without problem but suddenly some days ago when we tried to get authenticate via web-interface it didn't accept anymore our Windows user credentials. The only way to get access is to add also @domain.com after the "UserName" but before it wasn't this way. Moreover i've cheched on the local machine and i see an error that goes like this:

Faulting application name: VisualSVNServer.exe, version: 2.2.31.0, time stamp: 0x55e5b4c5 Faulting module name: libhttpd.dll, version: 2.2.31.0, time stamp: 0x55e5b4c5 Exception code: 0xc0000005

I've updated the VisualServer to the 3.9.2. We do not have the enterprise editon but only the standard so there's no single sign-on. The event viewer shows error about wrong name or wrong password. We don't understand why now we have to fill the user textbox with all the UPN format. Any help ?