Server lag

I have had Botania mod installed before, but for some reason I removed it. The world follows my use of /fml confirm to allow him to skip these blocks. I don’t know if this is a sequela. Now it only has Pixelmon 6.3.1 and some plugins installed.

The server will lag for a while, server console appears this message

[Server] Server thread/WARN Tried to add entity minecraft:item but it was marked as removed already

and server crash