If you haven't updated your version of Virtualmin/Webmin for a while and you encountered this error when 
you tried to update - e.g. from version 4.12 to the current version 4.17:

[Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 404 Not Found" Trying other mirror.

See the screenshot below:


It usually means the yum cache-files are stale, i.e. the location of the required files for the update
has changed. To fix this, run:

# yum clean all
# yum check-update 

That will refresh the yum cache. Once that is done, you can run update again from the Virtualmin interface
or simply run the update command from the commandline:

# yum update