What Causes This?
The actual causes of this problem are numerous. The bottom line is, '%5C' in a URL means that the backslash '\' got encoded in the URL. Somewhere along the way, either unintentionally or on purpose, a string containing one wound up in the encoding step.
The culprit for many users is using .NET's Path utility for trying to parse URLs. Here's an example:
Let's look at expectation vs Reality
This is because Path uses a field called Path.DirectorySeperatorChar, which on Windows is '\'... A bit backwards that Path can READ forward slashes as directory names, yet only output backslashes in the return value.
How Do I Fix This?
There are plenty of good solutions to fix this. However, with so many different causes to the problem, it's hard to say exactly.
To Fix it in a simple string
It's possible that all you need to do is Path.GetDirectoryName(your_string).Replace(Path.DirectorySeperatorChar, Path.AltDirectorySeperatorChar), and you're good to go.
Hopefully this shed some light on what the problem may be in your case. Hopefully, moving forward, Microsoft is not afraid to add a .NET feature like PathFlags or something so that this doesn't crop up as much as it currently does.