Hi Florian,
I don’t personally use owhttpd' but suspect modern web browsers ask for items not supported by the OneWire HTTP server, such as the bookmark favorite icon shown in you logs - /favicon.ico. The basic owhttpd is probably only HTTP1, with browsers trying HTTP2 & HTTPS.
For openHAB to access 1-Wire devices, only owserver.service is needed on port 4304. I don’t use owhttps.service, and found it unreliable for testing in the past.
Personally, I use owserver and let the openHAB binding auto-discover all OneWire devices. These can them be added as Items, and displayed on your sitemap, or graphed all accessible from a browser.
I also found probems with ofws, so use the command line client owdir for testing only:
sudo systemctl status owserver.service
owdir
owdir /uncached
owget /<sensor listed by owdir>/temperature
# if that doesn't work - check the daemon is running and accessible via the network
sudo systemctl status owserver.service
telnet 127.0.0.1 4304
I’ve run owserver on Fedora for >10 years, on Raspbian, and now on default openhabian using a RPi4. Avoiding owhttpd and ofws seems to work! 
The openHAB OneWire Binding works well, and auto-discovers all items it finds. The only bug used to be displaying too many digits in the logs, but that is fixed now:
2019-11-26 19:12:17.194 [vent.ItemStateChangedEvent] - owOutside changed from 5.04851 to 5.03358
I hope this helps!