Thanks for looking into this further! I agree that it should be related to the issue you referenced. I read through that thread just now and the issues reported there are very similar and even the same error message is mentioned there several times.
In the last post there Site Health: REST/loopback requests are failing, scheduled event failed - #28 by Dan0sz several links are given with potential solutions. I have a feeling those are in the right direction for solving this and might actually already point to a solution?
Unfortunately, I don’t have the required knowledge of these kinds of configs, so hopefully you can find out something on your end based on all this?