How to restore a previous version of a folder?

--b2-versions doesn't play particularly well with crypt unfortunately as the suffixes produces will cause crypt to skip over the files.

If you list the underlying bucket with --b2-versions then you'll see the crypted names along with suffixes of dates. What I recommend you do is copy the files you need to a local directory, remove the date sffixes, then point a crypt at that local directory to recover the files.