Hi Christian,
now the problem has returned. It happened once before on December 13, so I uninstalled Col Reminder, restarted my phone, reinstalled and set it up again, and then it worked until yesterday evening.
This morning, an update for your app came through the Play Store, and I was hoping it would include a fix for the problem, but that wasn't the case. Even after restarting, the tile is still gray and not working.
Frank.