"Insufficient collateral available in your margin" Polo margin trading problem

I get this message although it makes no sense. Anybody understand why?

I have 0.78 BTC in total in my margin account.
I opened short position on 2602 BTS at 6880 satoshi for a total of approximately 0.17 BTC.

Afterwards price drops to 6715 satoshi. Great. Just what I wanted.

Unrealized P/L says
0.00357446 BTC

Unrealized lending fees
-0.00000059 BTC

Margin trading account says:
Total Margin Value   0.72678003 BTC
Unrealized P/L   0.00345257 BTC
Unrealized Lending Fees   -0.00000059 BTC
Net Value   0.73023201 BTC
Total Borrowed Value   0.17530806 BTC

So everything is rosy.
Yet if i want to place a stop-loss buy order (in order not to be glued to computer) it says:

"INSUFFFICIENT COLLATERAL AVAILABLE IN YOUR MARGIN" - when it is clearly not the case.

What it is that I don't get?

Or should I at that to a long list of Polo problems?