Do you seriously expect that your viewer will be only used with same version host? Don't make that mistake. Viewer should be as much backward compatible as reasonable possible.
It is backward compatible enough to update the Host as soon as possible. But we cannot guarantee 100% compatibility when there is version mismatch simply because new features implemented in the newer version are not available in older Hosts (we cannot travel into the past and inject them in that host :) ).
If it is not possible to update the Hosts in reasonable time it is recommended to stay with the current Viewer then. It doesn't make sense to update Viewer only.
Regarding this specific issue - if you cannot update the Hosts then you should roll back to your Viewer 6.8 and try to reproduce the issue. We cannot troubleshoot this problem if there is version mismatch because we cannot know why it happens - that is whether there a "stand-alone" bug or it's an issue caused by version mismatch.
The problem is turned to be more generic. When old controller is unreachable, an attempt to connect to the member of the old domain randomly throws EAV between failed authentication attempts.
Please, try deleting the Viewer configuration file (config_4.xml) in the %APPDATA% folder. It will be recreated again once you run the Viewer next time. Let's see if it fixes the access violation problem.