Ideal Vogue Gen2 Combi Opentherm issue?

Hello All,

I am looking for some advice regarding my boiler and thermostat controller.
The boiler is Ideal Vogue Gen2 Combi, installed 4 years ago. Recently I upgraded heating controls to Evohome with Opentherm bridge. Since the Evohome was installed I started getting "Flow/Return Thermistor Fault" lock outs on the boiler - number of times a day.
I contacted the boiler installer who opened ticket with Ideal. Their engineer called in after few days and replaced flow thermistor. It was OK for 2 or 3 days and the fault returned. I can't get Ideal to come back to have another look again. I rang them myself only to hear it must be flow issue.
Anyway I replaced the opentherm relay with BDR (so ON/OFF control) and the issue is gone. The boiler is maintaining the flow temperature regardless of how many TRVs are open.
I am going to try to contact Ideal again, but was hoping for some advice before I ring them.
If my boiler worked OK for last 4 years with old thermostat (ON/OFF control), now works OK with Evohome with ON/OFF control, but doesn't want to work with Opentherm control (varying flow temperature setpoint) - in your experience is it pointing more at issue with the boiler itself, or could it be something with the heating circuit (flow)?
Please advise.

Thanks,
atom