Is this the same installation that has nearly 2,000 entries in the approval queue?
I think that's related, though it likely is still a bug. In XF1 we limited this to a maximum of 30 entries at a time. As it is now, it's probably hitting the max_input_vars limit in PHP.
As a temporary measure, you could increase that in php.ini, though you'll likely need to increase it to above 6000!