This had been caused by a bug in Optifine. Please update Optifine.
If you're on 1.8 or 1.8.9, no new optifine versions have been released which contain the fix for that bug. In that case, this old workaround still applies:
HD fonts on some systems are bugged. Try to remove either just the fonts from your resource pack or remove your resource pack as a whole.
To remove the HD fonts from your resource pack do this:
1. Open the resource pack .zip file
2. Go into assets/minecraft/textures and remove the font folder
3. Go into assets/minecraft/mcpatcher and remove the MCfont folder
If you're on Windows, there's also a video in case you're not sure how to do the above: https://youtu.be/In-VJrpF5UU