I’ve been moving my LaTeX to XeTeX these days and have had to cope with the error above for some time. What is happening? XeTeX is not able to load the particular glyph for the symbol \quotedblbase. Said symbol corresponds to the low double quote, which is the opening double quote in German. This inability of XeTeX in loading the symbol is due to the font seemingly not providing such a glyph in the given font shape (the shape would be regular here). XeTeX does now uses the glyph from the Computer Modern font, which is the default font in LaTeX, instead.
I tried now to make XeTeX use a different font, which is more alike to the one I’ve been using for the whole document, for this particular character. Thus making it unnoticeable, that the glyph is missing in my chosen font. You are able to accomplish this fairly easy in XeTeX using the fontspec package and the \XeTeXcharclass command. I am not going into detail here. Why? Because it is apparently impossible to replace the symbol \quotedblbase. I tried using the unicode character numbers (201E), the plain symbol („) and the command \quotedblbase, but it changed nothing and the error still appeared in my log.
The error seemed to actually be something completely different. The unicode character 201E, which really is the glyph XeTeX complains about, is not the symbol XeTeX is trying to load somehow. After some research I even found out that the apparently missing glyph is actually existing in the font shape I was using. So what was this error all about?
After another hour’s research I finally got to the point of solving the mystery. The glyph is not missing in the font. It is just some weird error of using csquotes, the package responsible for loading the glyph, and babel together when using XeTeX. When I switched to the package polyglossia (which is recommended for use with XeTeX instead of babel anyway), the error disappeard. And the quotes finally became the glyphs of the font I’ve been using and not the Computer Modern replacements.
Weird error, simple solution. People, use polyglossia with XeTeX instead of babel and safe yourself some unnecessary trouble!