Extra info:
The moment I cancelled the remaining QTUM buy order in Bittrex, Gunbot stopped giving me the error message and is now showing the 15 bought at the correct price.
For what I have seen, as long as there is an buy order in Bittrex which is not completely fulfilled and Gunbot isn't able to get the price for the partial buy, Gunbot also isn't able to sell it.
After the partial buy the price went up way more then my gain %, but no sell was possible.
Sven.