Sup, so whenever i want to join my server this comes up: http://prntscr.com/p6068s
Sometime it wont let me on server sometimes it lets me ._. but i cant see any effect on server.
So the error your getting is the world your attempting to join has had mods on it before and your now attempting to join without those mods.
The register of minecraft:dataserieraliser is from sponge, so you should be fine to join the world without any issues.
As you have stated that it sometimes doesnt let you join, whats the error?