Difficult to diagnose without the part (try to upload), but it might be due to some very small edge segments on your surface.
Try to use Untrim Surface first (perhaps with a percentage) and then Extend Surface as needed.
I doubt it's a permanent fix, but it's done for the moment (project is due in 2 weeks!).
Extend Surface > Untrim Surface
It worked and there are now no errors in the surface. Brilliant!