my exact same issue, it looks like
Just revisited this problem of yours and it would appear the program creating your flight plan is using the TAB character (ASCII 09) instead of the SPACE character (ASCII 32) to format the indented text. I've fixed this for the next release
I have news / feedback :
I did not solve by simply replacing, in any flight plan I tried, tabs with spaces through Notepad++
I then selected, in Load-flight-plan menu "Extended control sectors in use....", pressing "extended (I was not aware of that because I did not realize that I was still on 3.7 manual)
now, almost all flight plans are loading, although with a warning dialog "the last waypoint is too far from your destination airport............"
altough I am not sure, even by reading the manual, what function I have activated, now PF3 has started to work as expected