I have used Pterodactyl Panel, had the same issue server worked and ran perfectly fine was able to connect etc, same with a docker created by Matracey but had the exact same results, works fine out of the box but soon as you run a mod it throws up the error previously stated, I have spoken to the devs of Pterodactyl as well as went through quite a few different dockers and it all seems to be an unraid issue.
I normally run the servers from an linux setup with dockers and has always worked perfectly but from what I have researched an update on unraid around the start of the year seems to have broken them (seen the same issues been created on github from start of the year) as I have tried various ones