Another Linux Mint 20.3 user here. Appimage for 8.1 opens all right, 8.2 does not, or any later releases.
Running in console gives error messages like:
digikam: /lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /tmp/.mount_digiKayX0pb5/usr/lib/libMagick++-7.Q16.so.5)
digikam: /lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found
Thatâs a fairly low level library, so you might be able to install it separately (look for something like
libstdc++6). Afaik, that library is not the same thing as glibc, which a C library (and not C++).