Did you check your log for CSR: 429 errors which may indicate your certificate is stuck and someone from Plex would need to manually refresh it?
If it is that, hopefully someone sees your response, otherwise you can post in one of threads for it: Certificate stuck on 429 rate limit - request reset - #78 by BadFurDay
Edit: just read your item 6, if it’s a new server, it’s likely not this, but will leave it up just in case.
