Hm, it seems that it has something to do with Bazaar:
I had Bazaar installed previously for testing but it didn't work properly so I disabled the extension.
After reading Victor12 post I uninstalled it completely. After doing this, I had a lot of my test replies in an existing discussion. âī¸
Edit: Ok, this is getting weird. I reloaded the page and can't find those test replies...
Edit II: I use cloudflare but used the 'Clear Cache' button in the admin panel. No changes.