After the SP1 was installed, all Active-Sync clients (WM6 and others) worked fine.
Some days later I installed the additional SP1 service packs both for Microsoft .NET Framework 2.0 and .NET Framework 3.0. Then none Active-Sync clients worked, just error codes. Removed only Microsoft .NET Framework 3.0 and the active sync was back as usual.
Not sure what the SP1 for .NET Framework 3.0 does, but be aware.