I have just figured out what the problem is. I had followed a suggestion in the forum to correct unicode text by changing the setting in the Windows Control Panel for “Beta: Use Unicode UTF-8 for worldwide language support” to checked (enabled). This caused a problem in the customflight.pln file by replacing the degrees symbol with a strange character, which messed up everything. After reinstalling completely twice and not being able to fly from the planner for 2 weeks I finally figured it out.
Lesson learnt: DON’T enable this Regional setting, and DON’T blindly follow recommendations given in the forum. Write down your changes (with a date) so that you can retrace your steps if it doesn’t work.
Apologies to Asobo for being frustrated with the product whereas it was just a bad (and untested) user recommendation.