Can't install premium -- page not found error

I just purchased premium and was able to install it on my office system. But it will not install on my client's shared hosting system.
I get this error every time after clicking 'Install Now'.

OOPS! THAT PAGE CAN’T BE FOUND.

This is the url shown:
.../wp-admin/update.php?action=upload-plugin

update.php definitely exists so I don't know what's going on.

So I just installed the plugin manually with the file manager. Seems to be working.