Customers may report problems checking out or logging in when using IE11; in particular, the web site may seem not to respond appropriately when the 'new' or 'existing' user radio buttons are pushed by showing the email/password text boxes. If these do show, the site may not register the selection of shipping option, making it impossible to check out.
This is not a Kartris issue per se, it is a problem with ASP.NET 4.0 due to it not properly handling the latest Microsoft browser. Microsoft has a hotfix for ASP.NET 4.0. However, we've found this unreliable.
The working solution we have found is to install .NET 4.5 on the server. This is available for free from the following page:
ASP.NET 4.5 effectively replaces ASP.NET 4.0 and seems to fix problems recognizing the newest Microsoft browser.