access denied error with SPD workflow

Lately running into a problem when any user with "designer" permission level failed to save workflow from SPD with an error saying " Server Error: Access Deny". Further, users with designer permission can't manually start this workflow from browser with a similar access deny error. The workflow startup page is one of four files listed for each workflow in SPD, and it is missing from designer view:

Check out and check this file back in, the error goes away.