Hello TIG,I solved! Thanks for your help!
Looking at the Ruby Console I could see the problem and solve it by following these other forum posts:
https://sketchucation.com/forums/viewtopic.php?f=323&t=20485&start=675
https://sketchucation.com/forums/viewtopic.php?f=323&t=61213
This was the error message from Ruby Console on my desktop:
Error: #<Errno::EACCES: Permission denied @ rb_sysopen - C:/Users/PcCom/AppData/Local/LIBFREDO6_DATA_Dir/LibFredo6_all_defaults.dat>
...
...
I change write and read permissions of the entire %appdata% folder (it was not enough to increase permissions in the SketchUp's Plugins folder) and the problem was "easily" solved.
Thanks for your help and kindness.