Error when opping myself on a server

[nicky.gg]

So, I started a Forge server which is hosted by cloudnord (don't know it this information is necessary or not). Everything's working fine, the mods are getting loaded, I can join and play on it. Then I wanted to give myself OP. That's where the problems began. I typed "op mc_nicky_com" in the console and just saw, that I automatically disconnected... Weird, I thought and promptly updated the forge version to the client sided version, 47.2.17. Nothing resolved. Thought of downgrading the server to 47.2.0 but still didn't work. Now the server is on 47.2.0 and I can play without OP, which is totally fine but if I want to do something only OPs can do, I have to put it into the console, where I don't have auto-completion. The error it is throwing is the following:

io.netty.handler.codec.DecoderException: java.lang.IndexOutOfBoundsException: readerIndex(31613) + length(1) exceeds writerIndex(31613): UnpooledHeapByteBuf(ridx: 31613, widx: 31613, cap: 31613/31613)

I don't know how to fix this and I am thankful for every help attempt that comes here. 

With best regards

nicky.gg