You may need to update the server's jar file. For example, if your server runs on paperspigot, you may want to try an updated jar file or an alternative jar file. If this fails, you will probably have to delete all plugins (make sure you back them up) then install one at a time into the server and see if the issue still exists with that individual plugin in the server. If a plugin, let's say mob disguise for example, is the only plugin installed on the server and you experience this issue, then you may want to delete mob disguise or use an alternative build of the plugin (if one is available). Feel free to pm me if you have anymore questions, thank you!
~Spreading love and not hate one video game at a time. LGBT+ Gamers <3