So, that happened after you added MoarSigns (and its obviously from 9minecraft, baaaaad) to the server, right?
Your VERY old version you got off 9mc: https://www.curseforge.com/minecraft/mc-mods/moarsigns/files/2223209
The actual next version uploaded 3 days later: https://www.curseforge.com/minecraft/mc-mods/moarsigns/files/2223519
The actual latest version that's not from the start of 2015: https://www.curseforge.com/minecraft/mc-mods/moarsigns/files/2284576
