End of Season adds a significant burden on HIVE blockchain for few minutes.
Because most people would like to claim their season end reward chests at the same time, and the Hive blockchain cannot handle that yet. Hopefully the scalability of the Hive blockchain will be indeed better in the future. I also encountered a similar error message in the past a few times, but not nowadays, because nowadays I do not play Splinterlands (I am automatically leasing out my Splinterlands cards on Splex.GG).