BUG - No Proper Font in UI Windows

I'm completely new with Joplin and not very experienced with such forum issues in general.

Hope I chose the right channel. If not please let me know.

But I didn't found solution for my problem with bare google search. Therefore I try to get support within this forum.

At the moment I uses following configuration

Xubuntu 20.04 on a Levono Laptop
Joplin v1.2.6 installed as snap from snapcraft.io

Synchronization with an opendesktop account (Nextcloud). This works proper.

First the installation works fine but after some time (creating notebooks and notes / shifting between notebooks / ...) a failure rise up. When I try to create an attachment the font within the popup UI windows won't be shows proper (only squares instead of letters). Same when I try to export the whole notebook. This failure stays after program restart also.
And complete removal and new installation of the snap doesn't help.
Here the picture:

Can somebody help me to fix this proplem?

Thanks in advance