It started with a player on my server getting kicked out every time he went through a nether portal. Then, yesterday, the server started crashing whenever he went through the portal. Now, it crashes when signs on.
It's also giving these weird warnings on startup:
[21:59:46] [main/WARN]: Ambiguity between arguments [teleport, destination] and [teleport, targets] with inputs: [Player, 0123, @e, dd12be42-52a9-4a91-a8a1-11c01849e498]
[21:59:46] [main/WARN]: Ambiguity between arguments [teleport, location] and [teleport, destination] with inputs: [0.1 -0.5 .9, 0 0 0]
[21:59:46] [main/WARN]: Ambiguity between arguments [teleport, location] and [teleport, targets] with inputs: [0.1 -0.5 .9, 0 0 0]
[21:59:46] [main/WARN]: Ambiguity between arguments [teleport, targets] and [teleport, destination] with inputs: [Player, 0123, dd12be42-52a9-4a91-a8a1-11c01849e498]
[21:59:46] [main/WARN]: Ambiguity between arguments [teleport, targets, location] and [teleport, targets, destination] with inputs: [0.1 -0.5 .9, 0 0 0]
These warnings are the very first thing showing up when the server is started.
This means the server was running too slow and therefore was shutdown. Try the solution described in MC-63590.
The "Ambiguity between arguments" warnings are caused by MC-124672 and you can ignore them.