I have just updated one of our development portals from 10.3.1 to 10.4. The portal comes up OK, but when trying to switch between top menu items, the following error is thrown in Internet Explorer 11:
A potentially dangerous Request.Path value was detected from the client (:).
We are running our portal (and IIS-based web adaptor) on Windows Server 2012 R2. Some research indicated that ESRI moved to .NET 4.x at this release, which has more secure request validation in ASP.NET.
The good news is that this error only affects IE configurations that have Compatibility View enabled. The bad news is that many organizations (including ours) have this turned on by default for intranet applications needing earlier versions of IE. I have logged an ESRI case on this, but if anyone experiences this error, turning Compatibility View off is the workaround.