Error when migrating data from internal microSD to internal storage despite apparent available space
Despite the SD card was showing as 1.78GB used out of 1.80GB, it was already empty. So, I formatted as portable and ejected without data loss.
As I reported in my question, there were no apps or cache stored in it. It showed as used because it was formatted as internal, although it didn't have any data in it. About the option to migrate data back and failing when attempted, I think it's some buggy interface as this is an old Android version and the first one to provide this feature (internal storage).