For several days I was unable to connect to any Cloud9 instance, even instances shared with my colleagues that they could still access. It would start the instance, but then hang on "Connecting", eventually giving me a message that it was taking too long and that I should contact support.
In the browser console I had a lot of errors about "CORS request did not succeed".
Support helped me figure out that there was nothing wrong with the VPCs or any other settings. We came to the conclusion that there's something wrong with my network, since using another internet connection worked fine.
After a bit of debugging I found out that my ISP had an issue with their DNS lookup. Switching from their default lookup to 1.1.1.1 solved the problem.
Posting this here so that it becomes searchable for anyone else that might have similar problems.
