I had this happen to me... only thing is i can't remember how i fixed it. I know that I had to have someone help me navigate through the menus (since i couldn't read them) and then i had to change one of the settings.
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Could be anything in the chain between storage (indeed PNG) and eventual display, not just SDL. Maybe the OpenGL implementation doesn't handle one of 16-bit or 32-bit textures for some reason, or alpha blending was turned off (if the current font textures have all information in alpha), or something else that doesn't come to mind happened.