There were some issues at Athom yesterday evening and as a result I had the same problem (although Homey 2019). After liogging out and logging in again the problem was solved for me.
My Homey is connected via Ethernet. I unplugged my Homey for 1 minute, started up fine flows, and switches work. But cannot connect to my Homey using my iPhone app or web app.
I also rebooted my homey without ethernet, but did not make any difference.
I checked the App Store for updates for the Homey App, and there were. so I updated the app.
When I want to log in the app crashes.
So I logged out from Homey on my mobile app. With the hope of logging back in. Does not work, when I signing with my credentials I get an error message "Invalid response from the server: [Object object]. See the attached picture.
I am also not able to log in via the web app, after filling in my credentials click on login in it loads, but after 30 seconds or so I need to filling my credentials again.