Hello,
As previously stated this could probably be caused by a proxy issue, however, a couple of days ago when trying to do a fresh install on my personal desktop (behind no proxy, checked internet conn.) this exception was shown a couple of times (couldn't reproduce the issue using another home-pc however).
I solved this by installing and configuring firewall permissions (and some admin privileges) for tortoise-svn with command line before exec. the opentelemac automatic installer and unchecking the corresponding pre-requisites.
Off-topic: During that day i also ran into small issues related to path's variables, mainly caused by previous installations of Python and MingW64...
Regards,
José C.