@j-ed Thanks for the suggestion, but DAVx5 is selling this app as tested with Synology Nas
My installation is exactly as stated in the set up instructions, so DAVx5 should be able to diagnose the problem.
You can imagine the Synology staff probably won’t be interested in supporting a third party app that doesn’t even run on their hardware.
Well, your interpretation of ‘tested-with’ probabely does not take the biscuit. It’s not about selling any Synology app such as CardDAV server with DAVx5, but more of indicating basic compatibility. And in fact, Synology CardDAV server works in general with DAVx5 which I can confirm from my own experience.
Second, DAVx5 cannot be made responsible for some strange behaviour (e.g. redirection) outside the scope of DAVx5 and the expectation ‘should be able to diagnose the problem’ is of course desireable however not very realistic.
Third, using a Synology forum similar to this one is not about bothering Synology staff, more about asking other Synology users.
Back to the issue, redirection may occur for several reasons. So there are several potential places to cross-check. Since you logs show CardDAV standard ports for http and https, it would be worth to have a look at the CardDAV settings for redirection from http to https. Furthermore, you might also want to check the DSM reverse proxy for potential redirection of the port in question.
However, the easy way out might be - at least temporarily for testing purposes - to deactivate http to https redirection in CardDAV server and setup any device with https.