It sounds like replication on your connection servers is out of sync. Have you tried running the command in the error to confirm replication is working? Regardless, I would perform the steps in Restart order of the View environment to clear ADLDS (ADAM) synchronization in Horizon View (2068381).
Are you saying you already upgraded your internal connection servers to 7.5 and have additional external connection servers in the same pod that are on 7.1 and failing to upgrade?
Yes i did preform those steps as well as forced replication many times. That is exactly the case, internal went to 7.1 > 7.5 no issue, then upgrading the external to 7.1 > 7.5 failed.
I saw a similar issue on 1 of our 4 connection servers when I upgraded from 7.1 to 7.4. One failed and we had to uninstall/decom the connection server on it and reinstall it working with VMware support.