I tried SFTP Net Drive as suggested. The reported versions are sftp 188.8.131.52, driver 184.108.40.2066.
Plugging USB flash drives is fine, no reboot.
I forgot to mention, that die reboot only occours, when a crypted folder is mounted.
And I encounterd a reboot on yesterday, when editing a Ms-Word document on a boxcrypter drive.