In fact, after deleting all the Docker-Subvolumes, I get
Failed to import any pool on this device(sdc). Error: ‘unicode’ object has no attribute ‘disk_set’
So, I would think that there is something wrong when importing an existing pool with many (maybe with nested) subvolumes. On top, there seems to be one more error (the one two lines above) and which I already reported here Warning! Disk serial number is not legitimate or unique
Greetings,
Hendrik