We rely on your GitHub (or BB) permissions. I believe that you need to have write access to the project. If you have that access and are getting a 403 - it’s likely either your permissions changed on GH (or GH is effectively down, as it is currently is I write this).
Based on your report, it looks as if you have permission to follow Project A (but frequent requests can sometime trip up the system). It appears you do not have access to Project B. If this is incorrect, then it is indeed a bug.
This is not our ticket interface. This is a Discourse instance we use for our community support. We use it for Feature Requests and attempt to answer Bugs/ give an arena for folks to help themselves, especially those who aren’t ready to pay for engineering support. If you belong to a paid org, you can submit a ticket through our Zendesk ticketing service.