This is likely a plugin that is causing this. Most likely Nucleus as it isnt fully compatible with sponge api 8 (and never will be as it no longer receives updates).
Try without nucleus and see if the problem persists
Just tried it with the same sponge forge as yourself without any other mod and it works fine. so its likely another mod causing it. Sadly the only way to find it is by removing mods. I would create a server copy and then remove half of your mods (ensure that mods that require other mods are with those other mods), if the problem persists then you know its one of those, if not its likely in the other half