We have gotten the same error message when trying to upgrade from 5.0.0 to 5.0.1 via the web interface, and also from 5.0.1 to 5.0.2.
Both times, we have had to use the command line to upgrade.
In the web interface, clicking the 'Force Reinstall' link for the current version also produces the same error:
"Error: Failed to download release 5.0.2! Please try upgrade manually."
We are on CloudLinux 6.6, and previous upgrades via the web interface were fine.