[KevinH]

I am sure it is drive letter related but it should not be % url encoding related
as that is converted to a local file path before comparison.

So in URLInterceptor.cpp, the the following code snippet:

        QString destpath = destination.toLocalFile();

       ...

        // or the path must be inside the Sigil's MathJax folder                                                                
        if (destpath.startsWith(mathjaxfolder)) {
            info.block(false);
            return;
        }

Immediately after this snippet but before it reaches the debug warning that it will be blocked, it might be useful to add the following:

qDebug() << destpath;
qDebug() << mathjaxfolder;

I am guessing there is some drive letter case or leading / difference going on.