We are currently investigating why some players are stuck with the Derby tasks. In the image below you can see that the task is labelled "Completed" BUT on the right hand menu it says "Expired". If you can't select & start a new task (you get the text "Failed to Start Task") you need to:
1. Tap on the task that has the label "Completed"
2. Check that it says "Expired" on the right
3. If this is the case (i.e. you are stuck and can't select a new task) trash the task.
Beware: some may lose the task & points by doing so, but until we find the core of this issue this is the only way to proceed. Please let us know in this thread how it worked out for you, and if it did.
Image from user rayhahaha
Sorry for all the hassle and inconvenience! I'll keep you posted when we have something new to report!