Quoting Larry Clapp (larry@theclapp.org):
... Backends/CLX/port.lisp: text-style-mapping, which calls Backends/CLX/port.lisp: open-font, which for some reason is not listed in the backtrace.
As a workaround, you might be able to select a different font. Try changing clim-clx::*clx-text-family+face-map* so that it uses some other font family. Perhaps there are fonts that just work for you.
d.