try renaming c:\windows\system32\mfewfpk.sys or mfetdi2k.sys to old.mfewfpk.sys or old.mfetdi2k.sys and rebooting.
Sounds like it could be a conflic with the minifirewall driver, and I believe I have run into this before with SBS.
We faced the same problem. We had to completely deactivate access protection on our SBS 2011! State-of-the art virus protection ...