We encountered this problem when editing a report with a PowerPoint template. The report is on an NPrinting 17.4.1 server. When opened with an NPrinting 17.3.1 Designer, this error is encountered:
Invalid length for a Base-64 char array or string
The solution in this case is to upgrade everybody's copy of NPrinting Designer. This is tedious and it would be nice to have a heads up or even better some forward compatibility.