How to fix 403 webpage access error?

Lately I have been fixing up my server a little after a broken debian buster installation. I reverted back to stretch simply because it works out of the box.
Now installing HA was a breeze, of course, but down the road on installing other “apps”, my HA sends me a “403 forbidden error”.

As far as I can tell, installing apache2 - needed for nextcloud - kills my HA page. I don’t have any idea where to look here. Other software, like emby or tvehadend, do run fine, so I am wondering in which ways HA builds it’s webpage and how installing apache2 can break access.

HA does continue to run. I can access it via my telegram-bot and run commands like restart off from it.

Does anyone have a clue to what is happening? Or how I can fix this?

If I am in the wrong Sub Section, please point me to the right one. I found this one most appropriate for my situation, since it’s not about installing but managing the webpage.