Any attempt to Save the current drawing fails with the error message One or more objects in this drawing cannot be saved to the specified format..
This AutoCAD error is usually related to custom objects used in the drawing (Raster Design, Civil 3D, custom add-on application), or to corrupted standard objects in the drawing database (a polyline with 1 or 999999 vertices, two objects with the same ID, etc.). Or you may have set the default Save format to a nonstandard value (see OPTIONS). Or you simply have not enough disk space in your drawing folder or temp files folder.
Check the disk space. Try to PURGE your drawing and retry. Or try to SAVEAS your drawing to an older DWG format (or DXF). As a last resort, you can also Copy/Paste your drawing to a new, empty DWG file. If you don't need to save (keep the latest changes), try to AUDIT / RECOVER your drawing file.
Always use the latest Service Packs (Updates) for your CAD applications.