Same here. It popped up multiple times while loading but then I was still able to log in and play.
After playing a few battles without issue I went to look at daily offers and then the game soft locked. After relaunching I played a few more battles and then tried the shop, which produced a server error and many of the usual items for sale were missing. Then I went back to the offers and it soft locked again. So it seems like the issue is somehow related to the purchasable offers.
Quick update: These errors have stopped happening now.
This is a generic error, most commonly caused by a connection issue.
This connection issue could be on your side or it can be anywhere between your console and trying to reach our servers (which have not been down again recently)