GD has to have TrueType support, and ImageMagick has to be installed; paths are in the "Paths" section of the UBB control panel for IM.

As for troubleshooting, only .ttf files (and the index document) should be in the images/captcha/fonts directory; i've seen some systems add files that can't be parsed and it breaks.

Also make sure the session, tmp, templates/compile directories are all chmodded 777.

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