Not BO but from have studied it a bit.
It's a regular occurance, just hit the "retry" button at the bottom. If timer resets, you're good, otherwise wait a bit and try a refresh.

Seems to be endchan doing backups or similar action, but without the code and/or hardware in place to eat connections whilst a lock is active, or similar strategies for atomic on-demand copies.

Happens a few times a day.