Attempting the FolderSync command, Exchange ActiveSync returned an HTTP 500 response (Internal Server Error)

Here what the error looks like in Exchange Remote Connectivity Analyzer:

Verify that both Basic and Integrated Windows Authentication are selected:

1. Open the Exchange System Manager.
2. Expand the Servers node.
3. Expand the server node.
4. Expand the Protocols node.
5. Expand the HTTP node.
6. Expand the Exchange Virtual Server node.
7. Right-click on the Exchange node and select Properties.
8. Click on the Access tab.
9. Click on the Authentication button.
10. Make sure Basic Authentication and Integrated Windows Authentication are selected as the only authentication modes.
11. Click on OK.
12. Click on OK.
13. For the Public node, repeat Steps 7-12​.

Make sure that the settings are identical in IIS:

1. Open the Internet Information Services (IIS) Manager.
2. Expand your server.
3. Expand the Web Sites node.
4. Expand the Default Web Site node (or website node that contains your Exchange virtual directories).
5. Right-click on Exchange in the navigation tree on the left, and select Properties.
6. Click the Directory Security tab.
7. Click the Edit button within Authentication and access control section.
8. Make sure Basic Authentication and Integrated Windows Authentication are selected as the only authentication modes.
9. Click on OK.
10. Click on OK.
11. For the Public node, repeat Steps 5-10.

FolderSync should now succeed in remote connectivity analyzer: