The server threw an exception and was enforced to shut down when players attempted to get into end portals. The error log is attached. 

Worked fine with 1.20 and 1.20.6 builds. Also worked fine with the 1.21 vanilla server core from Minecraft official site.

Edit: Tried to run the server with Spigot without any plugins after successfully got into the end using a vanilla core. As there was a player in world_the_end, the server crashed again with the same error message.