Okay Josh-H and I have discovered that the date format we are using (dd/mm/yyyy) is causing this issue. When I changed my date format in the Control Panel > Regional Options to (mm/dd/yyyy) on the client machine we were able to log back in successfully.
Unfortunately this may cause other issues with date-sensitive software.
Am looking forward to the next patch, but I suggest you call it a "hotfix" like Microsoft does - that way it'll look like an improvement to an existing feature!