Hi all, I could obviously solve the problem. It was not, as formerly guessed, the /etc/hosts file, the reason was found in ~/.cache/*
In ~/.cache/* I found several old files of documents (*.odt, *.ods). The customer told me, that not all files could be opened. Some opened fast, others real slow (as described before). How I worked (for those, who are interested): 1. So first I created and tested with a fresh user = all ok = knew now, the system itself is ok. 2. copied ~.config/libreoffice* from this user to the old user = no success, so knew, it is not a libreoffice config problem 3. searched for all files "*.odt" and "*.ods" in /home of old user = found some in ~/.cache/ = alerted, that they might interfere, so moved it away 4. New testing shows = all ok 5. copied all "*.odt" and "*.ods" to a Test-directory, marked all and opened them with libreoffice = no errors, knew, all files are ok. 6. Removed the testuser, all copies and the backup of ~/.cache Hope, this might help others, too. Thank you all for your help. This case can be safely closed. Best regards Hans