I think the 190d has a slightly different head layout because I hit the same problem (on a Sunday afternoon) on the last head bolt in the center back.
The casting comes up here just a little too much for good contact.

My solution was to drive the torx head out of the socket just a little bit with a centerpunch, enough to get the length needed. It wasnt too tight and just a few taps did the job. Definitely clean the head bolts carefully, even if it takes all day, and if you round off the torx bit, grind it flat again.