I wanted to pass along another tip for those who still can't fix it using
@hmmwhat's solution.
With Apple being so clueless and helpless about the issue, I decided to downgrade while trying to keep my data. What worked for me was:
1. Back up the phone on 11.2 (just in case I had to go back after all).
2. Downgrade to 11.1.2 using the "Update" function in iTunes. I didn't know that you can trick iTunes into letting you "update" to a lower version if you hold the Alt button on a Mac when clicking on it. I had only done this via "restore" in the past but then you'd loose all your data. So after downloading the 11.1.2 firmware and using the update function in iTunes to downgrade to this version, the issue was gone as expected but all my data was still there. However, my Apple watch wouldn't connect to the phone anymore and all health-related apps only gave me a spinning wheel when I launched them (activity, health app, sleep tracking apps, etc.).
3. I found someone with the same problem in a different forum and followed their advice. Since I already was on 11.1.2 with all my data intact except for the health apps working, I made a backup in iTunes. Afterwards, I restored to 11.1.2 via iTunes (this time using the "restore" button) and then restored my backup. This brought all the data back to the phone and allowed me to re-connect my watch. The watch wouldn't pair and I had to reset and delete all settings before setting it up as a new watch (so I lost my data and settings on the watch once again) but I'm finally back to where I was 1.5 weeks ago with a working camera and a working watch.
I've never had to go through so much hassle with any iPhone before and sincerely hope Apple will get their **** together and fix this in 11.2.1.