In the middle of a banner. Tomorrow is mission day. Press start mission but gets stuck. Says action in progress. Reinstalled, cleared cache etc etc. Nothing happens
This is a server side bug that sadly cannot be resolved by yourself. Normally, after 4 hours or so, it automatically goes away, so right now, you can only wait :/ It should be gone in time for Mission Day tomorrow though!
Same thing today. (second sunday January) Two Missions through, Lunch break, then the no-start issue. Not sure I can still be around in four hours and probably miss out on the badge.
Only known fix is just wait 4 hours, no other fix from Niantic.