Thank you so much for pointing this out, this issue affected a large number of other users.
The exact error, verbatim from the log:
POST Content-Length of 12143120 bytes exceeds the limit of 8388608 bytes in Unknown on line 0
What was happening (to ALOT of users it seems, based on my error log), is that the file, which is sent in the HTTP post request, was to big for the post request limit set by PHP in the PHP configuration.
I swear I had expressly set the value to be big enough, but apparently it didn't work.
It has now been fixed.
The learning takeaway for me has been to actually test if config values have been updated, since sometimes something goes wrong in writing the new config.
Sorry for any inconvenience this may have caused.