I've had this trouble for the last 2-3 weeks - but only if I try to MFU (WT+ to Club) the return on a Thursday (LHR-DEN-LHR) for Sept 22nd 2011.
Any other day in the week works just fine, but all Thursdays in Sept get the error message.
I'll wait for next sale
anyway, though it would be good to get the problem fixed.