Thanks DrYak. Yes, I bought the card at a big retailer in Germany. It has been working quite a while in my old Nokia N8 (filled almost completely with music). For this, I formated it with this tool.

With my Sailfish X I think I tested pretty much every combination (MBR and GPT on fat32,ext4 and btrfs). I just checked GPT & ext4 once again. It still does say "Memory card not inserted" in Sailfish. I worked with different tools (mostly with GParted started as USB Live system from stick, but also with several Windows tools).

To your other questions (check "access rights (by default root)", "what does 'mount' terminal command say" etc.): Unfortunately I don't know how to do this at the moment. I am new to the Sailfish world (since yesterday), and I'm new to Linux as well (coming from Windows...). I will take this as a challenge and explore this in the weeks to come, but this will take time.

But I'm glad for having with Sailfish a real alternative to Android and iOS - a system which is not based on monetizing private data...