Thank you so much for your prompt reply Fredo!
I went through all these motions several times actually: downloading, and installing Libfredo6, and Roundcorner (all newest versions, etc...) and was still getting the same problem...
My error was in how I was improperly attempting to proceed with updating by simply installing the latest plugin.
This was not overriding (any of) the previous version(s).
I finally just used the sketucation plugins uninstaller feature: uninstall roundcorner and libfredo6, quit, relaunch, reinstall Libfredo6, reinstall Roundcorner.
All seems to run normally and excellently now!
Thank you again.
edouard