I test navigraph with a VFR flight plan. I was able to export this plan in .pln format and recovered it in FS20 without problems.
But I use PF3 ATC, and there when I import the same flight plan PF3 puts an error message see attached image.
I contact PF3 and the designer replies this:
Your flight plan is not in the correct format.
As you can see in the error message, the standard end of line delimiters (0D0A - carriage return and line feed) are missing.
Check your Navigraph settings… I don’t use it so can’t help you
with this problem.