From what HCX version you are upgrading ?
What is the version you get on hybridity page when checking for upgrade?
AFAIK..Currently HCX 4.3.1 files should not be available and you should only see 4.4 for upgrade available.
Worst case you can try to reach out to support and provide ur SystemIDs so that they can reach to the backend team to enable HCX 4.3.1 .
That's odd.. Hope there is no connection issues to connect.hcx.vmware.com and hybridity-depot.vmware.com. Try a reboot of HCX Manager even and check. Make sure to check for any space issues on HCX Manager.. /common should have at least 20% free space .
Upgrade Cloud side first and then Connector site.
curl -k -v https://hybridity-depot.vmware.com:443
If using Proxy :
curl -k -v https://hybridity-depot.vmware.com --proxy <proxy-fqdn-ip>:<proxy-port>