Comment # 7 on bug 1179629 from Sebastian Wagner

This is the upstream bug report: https://github.com/flatpak/flatpak/issues/4043

It runs down to the issue that, by default "localhost.localdomain" is the
hostname and openSUSE-specific behaviour in some xauth-related stuff causes
troubles with the dot. Hostnames with dots cause that flatpaks based on KDE
won't run (sounds crazy, yes)

According to
https://github.com/flatpak/flatpak/pull/4130/commits/fd6a3b79f4e022e3f41f0c71ab7c6f21393c0db1
there's a fix in flatpak's versions 1.11.2 1.11.1 , but openSUSE Leaps' version
is 1.10 AFAIK (both in 15.2 and 15.3).

Hope this helps.