When a server has a scoreboard file with total size over about 2.2 mb, no one can join the server without getting kicked with the message in the attached screenshot.

This has been tested multiple times, and shown to be the scoreboard file. When the file is swapped for a different one, the server becomes joinable.

To reproduce:

• Get a server
• Put enough entries in the scoreboard file that it goes over 2.2 mb (such as putting a large quantity of mobs on a team)
• Relog

Note:
A rather common cause of this is MC-92266.