Thanks for trying to help.

I already solved the problem in a different way: I booted in the single-user mode (as described on RedHat's webpage) and deleted a few bigger files. After that, RedHat could boot again as usual.

I'm working at the command line quite a bit and using shell scripts (for analyzing research data). But I'm not very experienced.

Probably the reason why the 'rm' command didn't work ('permission denied') was that the big data were saved in home/username. If they were in /usr/local, I would have been able to remove them.

Anyways, thanks