MSSQL server is not taking backup of database after changing default port.

Pre-requisites for DB Backup and Restore process is that your SQL Server is properly configured in HC panel and that DCom Configuration kb article has been followed on both your HC Control Server as well as on DB Server. Link for DCom configurations is given below :
https://portal.hostingcontroller.com/kb/a262/sql-server-database-backup-remains-in-queue.aspx
 
Also, make sure that you have changed the port properly in MSSQL Configuration Manager against all the ipaddresses on the server. In addition to it, if you are having the issue only with the existing databases then make sure that the updated port is specified under InstanceName field in tblDatabases.
 
If problem persists then contact support team by sending an email to support@hostingcontroller.com