first off: having spaces in file/dir names is a bad idea in generall.
also check if the file is really not there, this error sometimes triggers too fast.
Never has really been a problem for me before.
Spaces should be fine.
Is "e:/dworld/maps/fel hills/fel hills_20_21.adt" really a location you can write to?
Yep, was a test directory I had made on a different drive to see if it was because it was being saved on my desktop before.
And, was still having that error.
Though, I seem to have somewhat narrowed down the issue. When I use texture replacement (change all textures in an adt from x to y) it is unable to save. So, not sure if this can help figuring out what's going on.