Try exiting LR and deleting the LR preferences: Preference and other file locations in Lightroom 5. On Mac 10.9 and later, be sure to reboot. Waving this magic wand fixes a surprising number of user-interface problems.
Thank you for your quick and promising reply.
I deleted the preferences, and (later) all other temporary files, but unfortunately nothing helped: Lightroom keeps its strange behaviour described above.
Did you try changing keyboards?
Another thanks for your further idea.
I just connected an external keyboard, and the problem still persists
I do not expect a hardware issue, as the cursor keys just work fine in grid view and all other programs. However, the hardware we are talking about is a Lenovo W540 Laptop.
Very strange. A couple more ideas with no particular promise: Try installing the previous version (LR 5.6 -- you can have both versions installed at the same time) and see if it happens with that. Also try reinstalling LR 5.7.
I installed 5.6, but also in 5.6 the problems shows up.
It's really strange, because I can say for sure, that the problem didn't exist a few weeks ago on the same computer in LR5.6.
Now it seems to happen independend of the LR version and also for fresh installations
I'm out of ideas. I don't recall other reports of this issue.
After hours of testing, I identified the reason:
The problem was raised by the service "skdaemon.exe".
It's meant to be a keyboard enhancing utility, which was preinstalled (und later updated) on this computer.
After deactivation (from autostart), LR works fine again, and I didn't notice any missing keyboard functions yet.
Maybe, this helps someone else in getting around that problem, and thanks for the support.
Glad you solved the problem, and thanks for posting the solution.
Do you have a Lenovo laptop?