Same issue here, was working all day doing challenges. Now constantly just getting the error. I've tried changing lineups, generating best lineup, closing and reopening, same error every single time. Very frustrating situation when challenges are the only things new players can really do without getting blown out by 99 overall teams in H2H.
I was able to get working by creating new lineup. Make sure you save your current lineup for online play first. I created new lineup and did not put the same players as I usually use, but it wont matter to knock out easy challenges, just put a starting lineup in to fill all gaps. I did this in the last half hour, and it did work, however I did use a lineup initially created from beginning of the year, that I saved, and filled in gaps. Hope this helps, as I too could not play challenges for the last week.
