just like [22:55:18] [Netty Server IO #7/ERROR]: io.netty.handler.codec.EncoderException: java.io.UTFDataFormatException: encoded string too long: 65550 bytes.
One of your mods or plugins is creating those files to store data.
Also, the bytes error is usually caused by a player trying a bunch of items that store items(ME Drives, backpacks) into their inventory. The easy way to fix it is to reset the playerdata.