One thing that I have found helpful is to make sure your backup and restore module for all your versions is the latest and probably best to be an Edge version. I’ve had pretty good luck but it has in the somewhat recent past miss some stuff from earlier versions.
I have never paid any attention to the built in default passwords in FreePBX. Things always worked and I have no idea if those passwords have changed from version to version. I would somehow hope they would. On my new box I just make sure everything is fully updated and then restore to it and fix what’s broken or didn’t restore. It is getting better! Good luck and sorry for the delay.
Unfortunately, this maybe a bug. A 403 error typically has to do with authentication/authorization. I’ve also seen a few others complain about this issue, but I’ll need to research it more to understand what the problem is exactly.