Oversized item or player data broke the server
What it looks like in your log
java.lang.RuntimeException: Tried to read NBT tag that was too big; tried to allocate: 2097152 bytes where max allowed: 2097152
What causes it
A block of NBT data grew past what the game will read: stacked shulker abuse, an exploit, or a corrupt playerdata file.
How to fix it
For one player, delete or restore their file in world/playerdata. If items are the cause, remove the offending container with a region tool and consider an NBT-limiting plugin.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log