In the meantime I have some more information to this:
The behaviour is the same as described elsewhere, where people had issues with the Win7 1MB extra boot partition. Once they deleted this extra partition the problem was solved.
However, I do NOT have an extra partition, but still have the problem.
My OS is on a partition that takes the whole 128 GB SSD. I tried to resize it, but with no effect. In the TC test run, the system always gets stuck with "booting ...." after typing the correct password.
I also tried to modify some bios fields (e.g. switch off Intel Rapid startup, changing to IDE mode), but nothing changed.
I have no clue, how to get this in order again (some time ago it worked, maybe it has to do with Win7 updates or new drivers, but which ones?).
Any people with the same problem and ideas for a solution?