Depending on your current configuration you might need to open the the API of your backup server. Check the Administrators guide for the paragraph of "IP Restrictions", it usually say something similar to:
To avoid AhsayOBS’s APIs from being exploited by hackers from an unauthorized network, AhsayOBS, by default, accepts and handles only the API requests originating from the same local machine. To loosen this restriction, please do the followings:
1. Open %OBSR_HOME%\webapps\obs\WEB-INF\web.xml.
2. Locate the ApiAccess filter under the “filter” section.
3. Change the [Range2] parameter value from “0.0.0.0-255.255.255.255” to 18.104.22.168
4. Add the [Range3] as well with IP 22.214.171.124
4. Restart AhsayOBS.