With recent updates to the machine image, the issue of hangs on the 'Scan QR code' screen has been greatly reduced if not fully resolved.
Previously, the machine process could hang at times directly after completing a transaction, returning to the Hello screen but thereafter preventing scanning a QR code for the next transaction. This would require a remote or in-person reboot of the machine.
With machine version 0.5.14 and up, we've patched the source of this error.
If used in conjunction with server version 2.2.10 and up, batch sending is enabled for transactions dropped due to local connectivity. See the article 'Dropped transaction support (cash-in)' for more details and instructions on updating the server. (The machine must be updated before the server is.)
Updating the machine
Please contact firstname.lastname@example.org to arrange an update to version 0.5.14.
The machine version is backwards compatible with prior server versions.