Having exactly the same problem, is all workng still with your vaillant wifi thermostat guys?
Mine Stopped working over last 24 hours. Have reset router 'virgin', turned off firewall, turned off repeaters so no mesh single router ap. Removed app and added again to both iphones I have. It seems to be really flaky logging into the app from various phones on different internet connections. All apps says is server is unavailable and then when setting up the theromostat when is has logged in the once - it complains of some workflow error on the app code 5?
Any ideas?