PowerPoint freezes and shows a Downloading box every time I open PowerPoint

The downloading dialogue box is for a file connected to the Express plugin that is either missing or out of date. The Express plugin may not be able to establish a connection.

Close PowerPoint and go through the following steps to resolve the issue.
The Express plugin may be attempting to use a Proxy Server to connect even if there isn't one.
To fix this, you will need to manually delete a registry key for the plugin so it forces the plugin to sign in again and then change the settings.
1. close PowerPoint.

2. click on the start menu and click on the cortana bar. Type Regedit and hit Enter.
(if you don't have cortana, you can click on start menu and do a search for "regedit" and it should start up.

3. when Registry Editor starts up, on the left tree column, expand HKEY_CURRENT_USER. under that, expand SOFTWARE.

4. Under that you will see a folder named "PresPro". Right click on it and delete it. Do not delete the folder named "PresentationPRO". only delete "PresPro"

5. close the registry editor and then start PowerPoint.

6. The signin form for Express plugin will load automatically. fill in the Email and password fields only.

7. Immediately under the password field is an option for Proxy settings. it should be completely empty. Click once on the Proxy Settings check box to put a check in it. Then click once on it again to blank it out. This time, all the fields below it should disappear. Now Proxy settings should be completely disabled. 
8. Click "Login" at the bottom. and you should now be able to use the plugin.

Other possibilities

1. If you are using a VPN, temporarily disconnect from it and open PowerPoint to see if that resolves the connection.
2. If you have a firewall enabled, temporarily disable the firewall and open PowerPoint to see if that resolves the connection.
    - if this fixes the issue, you can go through the specific applications that are blocked by your firewall and allow PowerPoint and PresentationPro Express.