In an XPM implementation, when saving or "updating preview" a call is made to http://CMSERVERURL/WebUI/Models/SiteEdit/Services/Services.svc/GetPreviewToken
.
This call is not returning any response (of any kind, success, fail, not found) and I've not found anything interesting (Errors or Warnings) in the Preview OData web-service logs (on Debug mode)
The web-service seems to be working correctly, as requesting /session/ws/odata.svc/Components
returns results