Trying to sell an ETF only traded in Berlin and AMEX

I use Flatex for trading stocks and I bought 16 shares of SPDR S&P Internet ETF on the Berlin stock exchange. It gained in value (~22%) and I’d like to sell it. For 2 months nobody wants to buy it (in Berlin). The only other place this ETF is traded at is AMEX. Flatex told me I can’t sell it there because the stock is stored with Clearstream Lux.

I realize the buy was a mistake and I should have done my research about the ETF. But how do I get rid of it?

Edit: Just to add that Flatex is absolutely useless in helping me with this. When I tried to sell on AMEX the UI only told me that I don’t have enough shares to sell. Took them 2 months to get back to my inquiry about the error message.

Edit 2: I put in an offer at market and after a looong time waiting I somebody bought took it up. I didn’t get the best price but a decent profit none the less.