Hi @Lodewijk,
I am not sure what caused this issue. I tried to add some extra logging but found that the main branch, which is version 3.1.0, is nolonger compatible with openHAB 3.0.x. So, I had to rebase the binding onto the 3.0.x branch and made a few adjustments to make it work again.
Anyway, during this rebase-work I somehow seem to have fixed the issue with dhw_mode (and I believe with ch_mode as well).
17:02:54.427 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'OpenThermGateway_BoilerWaterTemperature' changed from 48.5 to 48.91797
17:02:56.433 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'OpenThermGateway_DomesticHotWaterMode' changed from OFF to ON
17:02:56.439 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'OpenThermGateway_FlameMode' changed from OFF to ON
17:03:00.494 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'OpenThermGateway_RelativeModulationLevel' changed from 0.0 to 50.0
17:03:01.491 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'OpenThermGateway_BoilerWaterTemperature' changed from 48.91797 to 56.289062
17:03:05.493 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'OpenThermGateway_RelativeModulationLevel' changed from 50.0 to 54.0
17:03:06.473 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'OpenThermGateway_BoilerWaterTemperature' changed from 56.289062 to 54.777344
17:03:10.473 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'OpenThermGateway_RelativeModulationLevel' changed from 54.0 to 67.0
17:03:11.470 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'OpenThermGateway_BoilerWaterTemperature' changed from 54.777344 to 57.91797
...
17:03:13.475 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'OpenThermGateway_DomesticHotWaterMode' changed from ON to OFF
17:03:13.480 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'OpenThermGateway_FlameMode' changed from ON to OFF
17:03:17.446 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'OpenThermGateway_RelativeModulationLevel' changed from 67.0 to 0.0
17:03:18.446 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'OpenThermGateway_BoilerWaterTemperature' changed from 57.91797 to 54.757812
Can you please give this version a try?