Do you have Windows 10 Fast Startup enabled? I used to but it caused all sorts of problems in P3D for me so I disabled it.

Here's how: www.windowscentral.com/how-disable-windows-10-fast-startup

I didn't work by disabling the fast startup feature and nor did the re-installation, but I found this on the MD-80 Classic news page: coolsky.no/news/super-80-classic/368-sup...lassic-hotfix-v1-2-4 I think it might actually solve the FMS problem. If not, I will have to mark this topic with the exclamation mark.
Update: I need a special membership to go to the MD-80 Classic support forum, so this needs an urgent fix. I disabled Windows Defender, but everything is still the same.

/Viggo

To be clear, you are trying to load the FP in the Classic and not the Pro?

The problem got solved! I was so stupid... I thought that all I had to do was to simply request the flight plan and then just fire up the engines and fly away. But what I didn't know was that in order to get the flight plan inserted, it is needed to put in the flight number, the departure airport and the destination airport into the ACARS unit.
I am very sorry for this CAL. I should have read the manual before posting this thread.

/Viggo

Yep, I was trying to load the FP into the Classic one. Thanks for your help!

Not a problem. I'm glad you got is sorted out. Blue skies!