2 more reasons why the connection between SVM and DSC may fail, though ping from SVM Lif to Agent box, Agent box to SVM Lif is working fine.
While adding the DSC via SVM IP and vsadmin credentials, please check if the SVM Lif has is Data+Mgmt role enabled. If yes, please create a SVM Mgmt Only Lif and try connecting via this SVM management only Lif.
Also these kind of errors are seen, when same SVMs are added twice via 2 different DSCs by mistake. It will good to recheck if by mistake, the same SVM has been added by 2 different DSCs.