There are some people that are having problems with the Ikea Tradfri Gateway app and hub that suddenly, out of nothing, the lights can’t be controlled anymore and the message is ‘unknown_error’ when changing a light.
The only solution seems to be rebooting the app or the Homey itself.
I think i may have found a cause for this problem, it looks like something like a sleep or so.
I made the following flow and now the lights are working for 1 week straight, where normally i have to reboot the app multiple times per day.
If: every 15 minutes
Then: toggle (a random) ikea bulb on or off
This does turn off and on the bulb (in my case in the kitchen, every 15 minutes and seems to keep the connection alive(?)… idk, but it helps for me!