Well, I've figured out that it's worthwhile to understand how NFS and Kerberos deal with users, and how principal names come to play here - topics that are frequently left out in most guides.
In kerberized NFS, one needs to be aware of the difference between
the effective user executing the file operation
The effective (server-local) user of the file ...
So I never found an error pointing me in the right direction. However I suspected id mapping was at fault due my user showing uid 1000 and the uid on the home directory showing 1000 too. After messing with config files and rebooting the server and client several times I solved the issue.
Add the following lines to /etc/idmapd.conf on the server ...