Selected Answer
It seems to me like someone has changed some links and re-saved the document to which you are trying to link.
It's a dangerous situation when multiple people are able to edit documents on a shared server and there are formula links between the two files.
What I would do is to reset the links and make sure they work and then lock the cells that have the links and also those to which you are linking. Make it so that can't be changed.
If you need the user to be able to change source data, then let them do that but use another cell within the same workbook to reference that source data and then link to the reference cells and not directly to the source data. This method can also help to preserve links.
Update:
Try this:
Place the GETPIVOTDATA function in the same file as the PivotTable (in a blank area of the same sheet, or in another sheet), and then link the destination file to the appropriate cell [the cell that has the GETPIVOTDATA function in it].
I got that from this link on the Microsoft website: GETPIVOTDATA Issue
See if that works. The link says it is for an older version of Excel but give it a try anyway.
Update 2:
In addition to putting the pivot formulas in the worksheet that contains the pivot table, you can manage all of the formulas and functions that you want to appear in the linked workbook by mirroring those reports in the source workbook that contains the pivottable.
Simply mirror the worksheets that currently have the formulas, as in, copy everything, or at least the desired formulas, onto the same cells in an empty worksheet in the workbook that has the pivottable. Give that new worksheet the same name as the one that used to have the formulas; it will be easy to reference the data and you just have to remember to go to the source data workbook (the one with the pivottable) to update any formulas.