Crash: "Watching Server" - java.lang.Error Watchdog

Got this crash on every login in a 1.20.4 server that had been working for at least a month. Not sure why it suddenly crashes.

Upgraded to 1.20.6, same problem (but better backtraces in the crash report, at least).

I know "Watching server" means the server is too slow/busy, I'm not sure how to find out why from the log...

[12:43:37] User Authenticator #1/INFO: UUID of player Deefour42 is b73d2953-4d7a-403b-a40f-c26b743dbf6a
[12:43:38] [Server thread/INFO]: Deefour42[/192.168.0.2:44752] logged in with entity id 85 at (-82.07972271115337, 68.875, -36.54182046664898)
[12:43:38] [Server thread/INFO]: Deefour42 joined the game
[12:44:39] [Server Watchdog/ERROR]: A single server tick took 60.00 seconds (should be max 0.05)
[12:44:39] [Server Watchdog/ERROR]: Considering it to be crashed, server will forcibly shutdown.

 

---- Minecraft Crash Report ----
// My bad.

Time: 2024-06-02 12:44:39
Description: Watching Server

java.lang.Error: Watchdog
       at com.mojang.datafixers.optics.Traversal$Instance.wander(Traversal.java:41)
       at com.mojang.datafixers.optics.profunctors.TraversalP.traverse(TraversalP.java:47)
       at com.mojang.datafixers.optics.profunctors.TraversalP.first(TraversalP.java:57)
[...]

crash-2024-06-02_12.44.39-server.txt