Many users in Russian Wikipedia including me getting this error while trying to browse the watchlist. I have about 10 000 articles in this list. The problem was temporarily resolved after deleting all special pages from the view. Next day the error message was displayed again with shortened list.
[WXKeIgpAMCYAAFuSjlwAAAAP] 2017-07-22 00:37:50: Fatal exception of type «Wikimedia\Rdbms\DBQueryError»