HTC Sensation won't boot but vibrates regularly
1-2 weeks later, I wanted to rescue my data, but again it went into the vibration loop. I replaced the back cover with the one from the other broken device I mentioned above and this helped. Once it booted with the replacement back cover, I tried to reproduce the problem with the original back cover but now it boots ok with that, too.
Anyway, the conclusion is that the phone isn't completely lost when this vibration loop occurs.