Simbrief automatically found the directory and both the
Previous Flight files are stored in that folder. I tried to move the flight plan to C:\Users\my user\Documents\Prepar3D v4 files, but with the same results. So OneDrive is apparently not the issue.
Maybe the issue is the updated gauge file? I will restore the old and see if the problem gets solved. Hmm... It might also be the cause to the bug where I can't deploy the aistair.

Update: Nope, the gauge file isn't causing this.

Maybe I should just go ahead and re-install the package.
/Viggo