No, it is not mandatory to sign the configuration profile. Try the following steps
- Connect the device to the laptop (windows/mac).
- Open iPhone Configuration Utility.
- Create a profile of your need.
- Try installing it into the mobile.
- Check the console for error.
- If installation succeeded then export the profile and place it into the web-server and try installing the profile OTA. (Note: Uninstall the previously installed profile).
- Open the profile and analyse it for identifying your mistake.
Your research is right, the main reason for Unknown Error is the formatting mistake. The detailed description about the payloads and keys are available under this link.