I had the error one day, The reason I recieved it was because I reinstalled all the slackware packages...
"No fonts found; this probably means that the fontconfig
library is not correctly configured. You may need to
edit the fonts.conf configuration file. More information
about fontconfig can be found in the fontconfig(3) manual
page and on http://fontconfig.org
So what I did was I read the comments in the /etc/fonts.conf and added the root directory ( / ) as one of the directorys. Now i checked and fc-cache looks at the subdirectories also. So I did fc-cache as root and tried to run many gtk programs, but it still doesn't work.
Any suggestions on how to fix this?