I'll respond to this myself. It's no bug. Setting the pattern property doesn't prevent me for entering illegal characters, but only mark the text if it does not match the pattern. I seem to have some coloring issue with my virtual machine/remote desktop settings cause I didn't see this red mark.

The problem with buttons/checkboxes that stopped reacting on mouse clicks was caused nby still having  illegal characters in the edit control. It seems that no focus change is allowed in that situation.