No propane, propane pressure too high or too low, or it's overheated.
FWIW - I found the Truma US is very responsive to issues with their devices.
Code:
#122, #212 Combustion air infeed or exhaust gas outlet closed.
Check for obstructions such as slush, ice or leaves
and remove from the openings.
Switch for gas shut-off valve open.
Close switch for gas shut-off valve.
Gas pressure regulation system defective.
Contact Truma Service.
Overheating protection has responded.
Switch off furnace and allow to cool; if necessary, fill the
water container with cold water. Check warm air outlets
and circulated air intake and remove blockages. Switch
furnace on again.