Adding 6.7 Vcenter credential in netbackup 7.6.1

Recently I have migrated our Vsphere enviroment from 5.5 to 6.7 , then we upgrade our Netbackup hardware from Gen 6 to Gen 7 with same Netbackup version. After upgrading the netbackup hardware I restore the catlog and file level backup and restoration are working fine but Virtual server backup having issues due to credentails .I removed and try to add 6.7 vcenter credentials in netbackup 7.6.1 and 7.6.2 (m Release) but it gives credentails error. But I tried 5.5 vcenter credentials it is working fine. 

is 6.7 Vcenter supported on Netbackup 7.6.1 or do they have any maintance release apart from 7.6.2 ?

Suppose if I want to upgrade netbackup 7.6.1 to 8.0 , is same (7.6.1) license applicable ?

is there any other way please let us know.