What should I do about this Alpha HE CB33 Boiler pressure error condition?
You drained and re-pressurized the system.  This introduces air into the system.  Section 5.1 of the manual indicates a very clear sequence of steps that should be followed to re-pressurize the system, one very important one of which is bleeding (venting) the radiators, one by one, from lowest to highest.  

This is done to get air out of the system that's trapped in the radiators.  Air is compressible, water is not, so having air in the lines will mess with pressure readings something nasty.

Bleed the system and check the neons again.