I am not sure what else to say.
It is a case of trouble shooting to see what the problem is.
Drag a copy of the mesh to the side and try adding contours to that.
If that doesn't work then try splitting that mesh in half and try adding contours to either part.
If the mesh is not to complicated, can you re-create it as a new mesh?
Something else you can try is to save and close your file - leave Archicad open.
Now go to the FILE > OPEN menu.
Don't choose from the recent file list, choose OPEN again so you get the open file dialogue box.
There you should see an option to 'open & repair' a file.
Choose that and go browsing for your file and open it.
That may correct any problem if it is a corrupted mesh - it may not as well - but might be worth a try.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10