After the third or fourth disaster with the recurring tasks I've decided to inspect them manually and found out many recurring tasks with corrupted recurrence like:

* Monthly on the {0} Sunday
* Every 2 months on the {1} Saturday

Needless to say those were never regenerated and I've missed a lot of them.

How did that happen? How do I recover?

I am extremely frustrated because an unreliable task manager is worse than no task manager!