Ferroli DOMIcompact F24B: Unknown error?

I just came back from a couple of days away from home (boiler not used for 4.5 days), and my boiler is showing error message "F-", which is not in the manual. Neither heating nor hot water is working.

Pressure is just over 1, which is normal for this boiler.

I've tried the following:

- pressing reset: It goes through the P1, P2 and P3 stages if I follow the adjust procedure outlined in the manual, but then goes back to the error message.

- Turning the boiler off and on again after 10 minutes: It goes straight back to the error message.

Has anyone seen that error message before and can advise me how to fix the problem?

Thanks!