"A Suitable error message could not be found" error while changing mastery page

So i was in my Gold 1 promos twice today, and lost both times because i couldn't change my mastries in champion select. for 2 whole minutes everytime i tried to change the mastery page it would say an error occurred. On my mysteries it also showed 5 extra mastery pages that didn't belong to me for champs i don't even play.

I dont mind losing my promos once, but when it happens twice in the same fuking day it's annoying AF. Can some1 tell me how to avoid this error because the only way i know how to fix it is by closing client and losing my promos. Same thing happened to me last week and i tried to create a new mastery page while in champ select and I ended up with 0 masteries. So salty right now