Thanks for the info, it turned out to be a port issue indeed. Because Z-connector was configured to auto-connect on startup, no error was reported. I just don't understand why Z-connector only reports an error for a wrongly chosen port on manual connection. On auto connect everything looks okay, green sign and all, but apparently that's not always true. After changing the port and disabling auto connect I can access the webinterface again.
Unfortunately now I've encountered another problem...
On the Thermostats tab one of my 3 HRT4 ASR-ZW's has disappeared from the list.
And the buttons 'Switch to automatic control' and 'Switch to manual control' don't function anymore.
To force all devices to manual control, I've deleted all climate schedules. Unfortunately this hasn't changed a thing. The 3 HRT4-ZW's are still in automatic mode, and unwilling to switch to manual mode again.
Do you have any ideas how to get the missing HRT4 ASR-ZW back and how to get the buttons functional again?
Thanks,
Pepijn.
Server has not completely started yet
Nevermind :-)
Unexpectedly I've managed to get the missing HRT4-ZW back AND get the buttons to function again, by deleting the devices configurations and forcing a re-interview! 
It was re-interview that helped.
The deta about manual/auto is stored in device data, which is rewritten by re-interview.
It's hard to say what happened with your HRT. We have to look on the problem to diognose it.
It's hard to say what happened with your HRT. We have to look on the problem to diognose it.