F9 is a rather generic fault code in many cases so although loosely related to pressure/circulation it could be indicating the onset of diverter valve breakdown.
If you can, remove the DV, disassemble and clean it, lubricate with silicone grease, reassemble and replace.
I have also seen F9 showing when the fan was shot, changing the fan sorted the problem (you would need an RGI to do this).
Its a generic fault usually something on the low voltage side. The boiler has locked out, but doesnt know why it did it. Im not a lover of glowworm software - way too sensitive.
If you can, remove the DV, disassemble and clean it, lubricate with silicone grease, reassemble and replace.
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