Vaillant Thermocompact 628e Delay Mode Fault 5.53?

HELP!!

My family and I have had 3 nights of freezing cold as our Vaillant keeps igniting then switching off due to fault 5.53 which the manual says is a delay mode due to lack of water in the sytem. However the bar is at 2 so this cant be the problem.

I dont get paid till the end of Jan so I cant afford a call out.

Secondly, all the radiators downstairs wont warm up, the towel rail only will which is closest to the boiler. Updstairs rads are all fine. I have tried swithching them off and airing the downstairs ones but the shoot out water, so there doesnt appear to be air in them.

Please can someone help before a very cold weekend!