Symptoms
The synchronization does not work on iOS, while the Android version syncs normally.
Cause
The problem is connected with the peculiarity of mobile applications (more precisely with authentication). If the Basic Authentication method is enabled, the synchronization will end with an error.
Solution
1. Make sure the method is disabled in IIS. In the IIS structure, select a site, go to the Authentication menu. Only Anonymous and Forms Authentications are enabled.
2. Make sure that the .Net Framework version matches the values in the table. You can determine the version by referring to the following article - https://msdn.microsoft.com/en-us/library/hh925568%28v=vs.110%29.aspx
3. Restart the website.
Requirements
Access to IIS, access to the list.