Process Control is totally dependent on the standard frequencies and timeframes provided by SAP in General Settings -> Key Attributes -> Maintain Timeframe frequencies/Maintain Timeframes. When creating custom timeframes, customers need to keep frequencies always active and timeframes always available if any task was previously created with a specific custom timeframe.
When a plan is created, it is dependant of the timeframe chosen. Everytime users open planner, all the tasks are validated according to the timeframe chosen on their creation.
Sometimes, An ASSERTION_FAILED dump is raised by the system highlighting class CL_GRFN_API_TIMEFRAME and method GET_FREQ when accessing planner.
If you face the symptom described above, you can check the following SAP note:
SAP Note: 1970216: ABAP Dump when accessing Planner
If it was not possible to find the missing or inactive timeframe, there is a wiki page which explains how to find it: