I’m starting to get dangerous because this BS is P!$$!ng me off royally.
I got the same scary red screen when trying to move onto the next 100 comments after post 1400.
My work around was to first clear out my cache.
Then, being logged out of FR, the default number of comments per page was limited to 20.
Then, I logged back in.
It turned out the problem existed somewhere within those 20 posts after post 1400. It was clear sailing from 1421 on.
Posts 1401-1420 shall forever remain a mystery. Good luck to you!