I have similiar problems with the "owl"error after flashing the custom file several times…
After reading the whole discussion i maybe mentioned your Problem:
The TeddyCloud-Server should have a different IP then the AdGuard-Server.
Screenshot DNS Server in FritzBox Settings and in the Adguard Rewrite Rule are both …29
Now to my problem 
Believe it or not… in my first flash I had a working Teddycloud Toniebox.
During my vacation, I played around with the box, my network in general and the Teddycloud and unfortunately I got a bit lost… but I can’t figure it out.
My current guess is the AdGuard DNS.
First of all, my setup:
ProxMox server with AdGuard Home, as well as Teddycloud (installed via the helper script : https://community-scripts.github.io/ProxmoxVE/scripts?id=teddycloud
I have now reinstalled everything except AdGuard (Toniebox and Teddycloud). Unfortunately, I get the message “Owl” after flashing. As mentioned, I suspect that the AdGuard Home DNS is the problem.
I noticed that an entry had landed on the AdGuard blocklist:
I initially released the domain again in the user-defined rules, but then the forwarding to Teddycloud did not work.
I have now adjusted the rule and added important:
(.249 = TonieBox, .62 Teddycloud)
||prod.de.tbs.toys^$dnsrewrite=NOERROR;A;‘192.168.178.62’,client=‘192.168.178.249’$important
||rtnl.bxcl.de^$dnsrewrite=NOERROR;A;‘192.168.178.62’,client=‘192.168.178.249’$important
However, the result is still blocked…
As mentioned, very exciting, as the forwarding used to work perfectly before a week…
I had already extracted the flashed.bin and checked the certificate (which i created with ./generate.sh): it shows teddycloud as name.
Does anyone else have any ideas? What options do I have to exclude AdGuard or the TonieBox in failure analysis?
I’m using Pihole instead of Adguard but I had the same problem at first and can confirm that a blocked rtnl.bxcl.de will lead to error code owl and what makes this problem hard to detect is that the Toniebox sometimes works and sometimes not.
You have to allow (whitelist) this domain globally in Adguard, see here.
@@|rtnl.bxcl.de^$important
In addition to that, you need the dns rewrite rules for your Toniebox. The combination of both should work. What do you mean with “forwarding to Teddycloud did not work”? This is independent from the allowlist.
After I wanted to start “fresh” again and wanted to install a larger SD card in the process… and broke the SD card holder in the process, in the meantime I bought a new Toniebox on eBay for 15€ and was parallel able to fix the old “broken” Toniebox with superglue the SD card holder again…
I was able to find the fault in the AdGuard: It seems that the quotation marks usually caused the problem.
Due to the block in the meantime, I had adjusted the filter rules several times and that’s probably where the error crept in. Now it works:
||prod.de.tbs.toys^$dnsrewrite=NOERROR;A;192.168.178.62,client=192.168.178.249
||rtnl.bxcl.de^$dnsrewrite=NOERROR;A;192.168.178.62,client=192.168.178.249
Learning: NEVER Touch a running system 
!!
