We were able to successfully post a similar link-topic on our test server. The HTTP packet (captured by wireshark) looks like this:
POST /d2l/api/le/1.2/8083/content/modules/24980/structure/?x_t={timestamp}&x_a={app_id}&x_b={user_id}&x_d={user_sig}&x_c={app_sig} HTTP/1.1\r\n
Host: {test_hostname}\r\n
Content-Length: 186\r\n
Content-Type: application/json\r\n
Accept-Encoding: identity, deflate, compress, gzip\r\n
Accept: */*\r\n
\r\n
JavaScript Object Notation: application/json
Object
Member Key: "StartDate"
Null value
Member Key: "TopicType"
Number value: 3
Member Key: "ShortTitle"
String value: MTF
Member Key: "Title"
String value: My Test File
Member Key: "Url"
String value: http://www.desire2learn.com/
Member Key: "IsHidden"
False value
Member Key: "IsLocked"
False value
Member Key: "EndDate"
Null value
Member Key: "Type"
Number value: 1
Here is what the JSON data looks like printed out as the UTF-8 string sent:
{"StartDate": null, "TopicType": 3, "ShortTitle": "MTF", "Title": "My Test File", "Url": "http://www.desire2learn.com/", "IsHidden": false, "IsLocked": false, "EndDate": null, "Type": 1}
I see no significant difference between our JSON and yours, so the problem likely lies with the packet that you're actually sending out. A 404 would suggest that the route you're using is not exactly as it should be (for example, there should be a trailing slash after 'structure' in the route).
It's also possible that you're running into a permissions issue for the user-context making the call. Notice that the role permissions for viewing course content (and structure) are different to the permissions for creating and managing course content. You should ensure that the user context making the call has a role that grants it the appropriate permissions for not only viewing course content, but creating and managing content as well.
If neither of these suggestions helps, then I suggest escalating to the next level of support (open an issue with D2L's issue tracker for Valence, or contact the Valence support email address) so they can help you out more directly and with more detail back and forth.