One of our Customers recently came across this problem;

Unable to Save As or Import DWG/DXF
http://knowledge.autodesk.com/support/inventor-products/troubleshooting/caas/sfdcarticles/sfdcarticles/Unable-to-Import-DWG-DXF.html

Issue:
Save as DWG, Save as DXF results in:

•     Clicking [Option…] button doesn’t open the dialog box.
•     Error message is shown when clicking [Save].
•     No file is generated.

Import DWG / DXF results in:

•     Nothing happens. (Open works by clicking [Options..] > select “Import” > [OK] > [Open].)
•     File is not imported.
•     No error message is shown.

Causes:
Inventor is not fully registered.

Solution:

    1. Press [Windows] key to open the Windows’ start page. 
    2. Type cmd Right-click Command Prompt and select “Run as administrator.”

3. On “Administrator: Command Prompt” window, type cd “c:program filesautodeskInventor 2014bin” > press Enter. Then type inventor /install > press Enter.

    

Wait for 1 minute. (Inventor will run in the background and then exit silently.)