4legs I haven't had the same problem you reported but over the last 6 months have had several BACK-SP bets return a 'invalid_persistence' error message which, according to my post on Gruss, means I was attempting to place a SP bet on a non-SP market, which definitely was not the case as they were all SP markets. That said we are probably talking about less than .1% of all bets I have placed.

It did appear to be a API issue so fingers crossed it was somehow related to the problem you discovered & is about to be fixed.