Hello Thomas Waling
Have you tried copying the file to another location or even another computer?
You could also try reinstalling or installing another version, it could be a bug.
I use here, the same way as you, with windows 10, but in docx, I never had problems.
Only have problems, when I tried to use it in a different way, encrypting with windows, but this was lack of experience, I overcame ehehehe.