Thanks for sharing your experience. You can also try restarting the server but you should not really need it for diagnosis.
The first thing you want to try is of course to make sure that the target BBB server is working fine using the demo page installed by default.
When you are trying to make the plugin to work for the very first time, we recommend testing with the configuration by default. If that works you can discard a bunch of things.If these two conditions are fulfilled, then you can proceed with changing the URL and SALT.
Now, if after these three steps you still have problems, the first thing I would check is communication between servers. Making a wget call from the Moodle server to the BBB server can give you a good idea about what can be possible wrong.