This typically happens if your currently selected aircraft profile has performance settings that make the airport unusable for it. E.g., you may accidentally have typed that a runway should at least be 10 km long where you meant just one km. All airports with a runway under 10 km in length (and that's ALL of them) will turn grey to show the airport doesn't suit your needs. Now the runway length is just one example, of course. I'd recommend you switch to another aircraft profile to see if that cures the issue.

Ok, aircraft was ok. All airfields checked.

After deleting cache and restart now all looks good.