Logstash plugin install error

Hi there.
I have an on-prem Elastic stack implementation that needed the logstash github plugin. This isn't installed by default, and my stack is in an OT zone so is locked from the internet. To get round this, I followed the instructions and installed it on another machine that did have access to the internet.
This worked, and I then installed the offline pack on the locked down environment.
But, the plugin doesn't seem to be doing anything!
If I replace the github plugin with the TCP plugin, data is received and is passed to ES. With the github plugin, nothing. No logs or anything.
I went back to the logstash-plugin tool to get the list and get an error as in the screenshot.

Now I can't do anything with the plugin manager and am wondering if this error is also stopping the plugin from working in my pipeline?

Any ideas?
Thanks
Nigel