I can't provide too many details, since this was reported to me by the trapped player and I was able to fix it by teleporting him out, but hopefully the crash logs can provide that information.

As far as I know, he entered the Ratlantis portal normally, and was stuck jumping between the Overworld and Ratlantis. During this time, the console was spamming the following message ""fetching packet for removed entity itemEntity[RatPelt] over and over until the server dropped to 0 TPS and crashed.

Crash log is linked below.
https://pastebin.com/HwjF8nMz

Fixed in 5e55839