I have provided the good and offending flight plans. The '91' failure occurs during the PF3 setup, long before FSX is brought up, let alone connecting to FSX. Having once worked for a software house, am well aware these are the most frustrating types of problems to trouble shoot. The 'sometimes it works and sometimes it doesn't'. As I've pointed out, the operating system vendor, ahem Microsoft, updates the system when they want so difficult to know 'exactly' what has changed going to the 'Creator's Update'. That's why I've provided the release information and am happy to try to figure out what release of runtime libraries I have if I know what to look for...as in what language PF3 was written.
The only conclusion I can draw is that I've never been able to load a long (over 1 page on the PF3 route screen where you specify altitudes and the end of the SID and start of the STAR) flight plan. However, not a guarantee. The attached flight plan KIADEGLL12 will cause PF3 to abend if I attempt to assign the end of the SID, start of the STAR or adjust the altitudes by clicking the Activate quick-entry mode check box. At that very moment, PF3 abends with a '91'. And it is less than a page, although right up to the limit. A word about the flight plans I've submitted. They mirror real world flight plans as provided on the FlightAware web site, when the airlines and their countries allow (UK?). None of these flight plans were 'made up' by PFPX.
Finally, all of these flight plans work with FSX:SE and by extension my aircraft. In this case I'm using the PMDG 777. The plane flies the route just fine. This problem is confined to PF3.
Hope this sheds further light,
George