Some observations which might be helpful:
1. Uninstalling and reinstalling the Reader package, after clearing all personal preferences, did not have any effect.
2. Using a fresh version of the PDF form did not solve the problem.
3. Firewall and SElinux are disabled anyway.
4. With a "clean" account on the same machine, the same problem occurs.
5. On a very similar machine (same OS, same Reader version) the operation is working properly!
So it seems to be something specific to this machine, but not specific to a certain account. It is obviously neither a corrupted Reader installation, nor a problem with this specific reader version, nor a corrupted PDF form. Now I am running out of ideas...
Thanks in advance