Had the same issue, tried everything to resolve but nothing worked. Found in another question on here that it's an issue with (CDM) evolution players, I took my Conor Gallagher out of my team and it now works.
I have the same with Rivals (squad battles or friendlies online work well). I have read some opinions that it could be because of some bug with Traiblazer Interceptor which is glitched so I tried to put my Traiblaizer Interceptor out of the team but it didn´t help. Later I tried to put out different evolution developed player and it worked!!! My case was Traiblazer Slide Tackler, I had improved Dorley and now when I found out he could be the troublemaker I noticed that his original card has 4 playstyles, but the developed card only one, which means he lost some!!! And I think this is the bug which causes falling of the game....
So EA, please fix it as soon as possible!!!! I believe the problem si a wrong developed Evolution player so please repair all wrong developed Evolution players !!!! (i think it can by any player in any evolution if something went wrong with the evolution, e.g. lost playstyles...)
I couldn´t have played Rivals all the previous week, so of course I had no rewards for Rivals... And now I can play but I cannot use my card which I made just because I want to play him in Rivals... Fix it soon please !!!
3 weeks without playing rivals due to some buck with no solution. Super Frustating paying all this money to not be able to play the game
