Now I removed my wallet.dat so the q-t software would be forced to generate a new, clean one. 

Then I sent 1 SILK to the new wallet from my Bittrex account.
Then I sent what was left of this (minus transaction fees) back to my Bittrex account.
It worked fine.

The problem is in my wallet.dat file.

I may have found some crash events in my debug window.
Would those data be helpful?