can't mount an encrypted Volume on an M1 Mac. Works fine on Intel Macs. Known Issue?

There's no known, general issue with encrypted drives for the M1 Mac.

The symptoms you're describing sound very much like a corrupted file system. Together with bugs in the operating system's file system code, it can lead to these issues. I would run a file system check on the drive when unmounted. You can do this from your Intel Mac.

You can run the file system check from the Terminal like this:

sudo fsck_hfs -f /dev/disk3

Note that this assumes the file system is HFS+ and that it is mounted as /dev/disk3 (as in your screenshot). Use Disk Utility first to find the actual disk number.

The problem could also be of a hardware nature. I would suggest trying to connect the disk using the exact same adapter to another PC/Mac and see if that works. As you're using different methods of connecting the disk currently, it is difficult to know if it could be a hardware issue or not.