I'm using doxygen (1.18.0) on a C++ project of mine. In my doxygen.cfg, I have:

TAGFILES += "cppreference-doxygen-web.tag.xml = http://en.cppreference.com/w/"
TAGFILES += "libstdc++.tag = http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen"

I have the files, and when doxygen runs, it seems to recognize this properly:

Reading and parsing tag files
Reading tag file 'docs/cppreference-doxygen-web.tag.xml', location 'http://en.cppreference.com/w/'...
Reading tag file 'docs/libstdc++.tag', location 'http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen'...

but then I get a huge number of errors such as the following:

error: file '/path/to/cppreference-doxygen-web.tag.xml:algorithm' not found
error: file '/path/to/libstdc++.tag:algorithm' not found
error: file '/path/to/cppreference-doxygen-web.tag.xml:any' not found
error: file '/path/to/libstdc++.tag:any' not found
error: file '/path/to/cppreference-doxygen-web.tag.xml:array' not found
error: file '/path/to/libstdc++.tag:array' not found
error: file '/path/to/cppreference-doxygen-web.tag.xml:atomic' not found
error: file '/path/to/libstdc++.tag:atomic' not found
error: file '/path/to/cppreference-doxygen-web.tag.xml:bit' not found
error: file '/path/to/libstdc++.tag:bit' not found

and so on.

Why is this happening, and how can I fix it?