You may see in Framework logs or the Mosaic Error screens (21.2 customers above) the error:
'IllegalArgumentException: DateRange Start Date must be before or equal End Date'
Looking at the 'Caused By' area of that error stack, you see to add to that:
'carepackage.logic.charging'
This is dealing with the Charging area of the Mosaic finance functionality
This error is typically caused when Mosaic is attempting to cancel off elements but have not done so completely.
Usually, this means that the tables that matter have elements that are in a cancelled state, but some of the auxiliary tables which are key for user experience still have live linkages to those cancelled element.
This may cause you to be unable interact with purchase step in ways such as resuming, saving, completing, amending, and removing services, and generating contributions.
You may see a clear error screen, but where there is a blank white screen, this kind of error will sit in the Framework logs.
This currently is remedied with a script fix to correct the data and tidy the cancelled elements across all the various tables. Please raise a new case online and reference the title of this article.