Actually. It was on purpose. The directories shouldn't be initialized. I think that you need to create an account that doesn't introduce that much from the system side to be able to do the fault isolation. That's why I suggested to remove all created files in the /home/testuser1 that -m introduces if the problem persisted. But it might be a good thing to start with adding a user with yast.
1. from yast 2. useradd -m 3. useradd without -m (remove the created files)
Thanks for the tips. All of them failed. If I add a new user and log in to it, the desktop either doesn't start up, or crashes, or is extremely slow (and plasma crashes soon later on) – and the annoying message about the 'io-slave' doesn't go away either. So it seems that my system setup is somewhat broken, and I can be happy that my (single) user account works as expected for my purposes, more or less. Eventually I have to completely reinstall 15.2. Werner