Cryptomator doesn't show /mnt, where I want a new vault

I am using Linux Mint and have tested the AppImage, FlatPack and am now running the PPA. All behave the same:
After installation, when creating a new vault, I want it to be on another partition, which is mounted under /mnt, but the Desktop App doesn’t show me /mnt and /media at all.
I tried to create a symlink but wasn’t successful with that either.
My Nextcloud is on a different partition, and I am happy for any solution!