my steps to temporarily fix were to delete my existing lineup entirely and create a new one manually.
I picked a different player for each position, even the specialty positions. I didn't bother with 2nd string choices... just all unique 1st string players. Then I was able to successfully exchange for the 93 AKA Fantasy Pack was giving me the 'server error' and dumping me all the way back to the first menu. Of course this crazy team I had just manually made wasn't the one I wanted to play with... I was just doing it to get past the error.
Next, I created a whole new lineup and let auto pick my best players again. It did. Again it screwed up the 2nd string by assigning the same player to multiple positions. I don't care about 2nd string because I don't intentionally sub in players, so I just went with it (and I had already redeemed the set I was getting server errors from).
To this day, I still get an Invalid Lineup error after completing each challenge, but it doesn't seem to prevent me from anything that i'm aware of. I still get all the xp, coins and rewards I should get, but an error screen does flash up briefly after every challenge but it goes away just as quickly as it showed up. /shrug If I hit an error I can't get past again I'll try deleting my auto lineup and just use the manual lineup to get past the issue again.
I've read some folks can't even start a challenge with the lineup that auto generates for them. My issue wasn't that severe... I could always start challenges and even redeem most sets--even while having the wonky lineup. it was just the one AKA set in particular that gave me the server error and I was able to workaround the issue and get the set redeemed by manually creating a valid lineup (plus deleting the wonky lineup entirely). I wonder if the unable-to-start-challenge folks also had a duplicate player in their first string.
