Hello. Everytime the plugin loads the following error is thrown, but it then says that the plugin is successfully enabled. I have not tested further. The current server software I'm using is this one and I tried using the Bukkit build 5.1.51. Tell me if you need any more information.

Just checked and it seems that SporPaper patched the PermissibleBase attachments from a List to a Set. You can check it here.

The incompatibility is known and attempts to fix it have been undertaken. But it’s not looking good as the changes are problematic.

Oh. Sorry, I didn't know. Should I close this issue then?

Just read the other issues. I'll just close this one. And thanks.

My understanding was that SportPaper were going to patch this at their end :)

Yup, read that. Just submitted an issue there to know how's it going on.