SSH login via IPv6 successfull while using IPv4 to the same host yields "Permission denied"
After things getting stranger and stranger (see the thread of comments in my question) I finally figured it out. First things first: The authentication process did fail in pam_access.so however not due to some misconfiguration in /etc/security/access.conf as it was suggested.
To understand why, we must look at the setup of this box in particular: It acts as a router toward IPv4 which goes natively over the PPP link and IPv6 which is over a 6in4 tunnel. Also this box acts as a DNS recursive resolver, and here it is getting interesting. I did configure the resolver in a way that IPv4 reverse lookups are resolved recursively starting with the IPv4 root servers and IPv6 reverse lookups start with the IPv6 root servers. This setup did work when I first installed it.
Now my ISP enters the pictures and people who don't understand, how DNS amplification attacks work. To make a long story short: I know for sure that my ISP messes with incoming DNS packets at random, i.e. some things must be resolved through their own resolvers for some time now, while resolving other DNS addresses recursively on your own works – the official reason is to mitigate DNS amplification attacks, but they're doing it wrong then^1.
Since I didn't want to largely change my setup I just threw my ISP's DNS resolvers at the end of my local DNS resolver as nonrecursive forward, so if the recursive resolving attempt times out it tries my ISP's resolvers. This works so far. But when I did configure this I made a small mistake: I entered the ISP's DNS resolvers to work only from hosts within my local scope, i.e. 192.168.0.0/16 but forgot about localhost, aka my router, which is the host I tried to SSH into, for which the resolver would not take the ISP's resolvers into account.
pam_access.so attempts a reverse lookup on the peers address; and this closes the circle: Because for IPv6 reverse lookup the DNS IPv6 root servers would be accessed the packets would go though the 6in4 tunnel without my ISP messing with them, getting a response. But IPv4 reverse lookup would not be done over the ISP's resolvers by my own, which would receive no response and would ultimately report a NXHOST (or it would run in a timeout). Anyway pam_access.so won't see something it likes and just says "you shall not pass".
After I fixed that resolver configuration everything now works like a charm again. But I really have to step onto my ISP's toes now…
As to how I did resolve it? Well, by yanking up logging verbosity intensely studying /var/log/everything to see in which order things unfolded. When I saw my resolver logging the reverse lookup attempts I knew what was going on.
1: from a DNS amplification mitigation point of view this is complete nonsense, because I did test and outgoing DNS packets get through just fine – however those are the packets they should filter. In fact every end customer ISP should drop all UDP packets which sender address doesn't match those of their customers