I had the same problem as well, I checked my functions file and didn't see that I'd ever made any customizations to it, so I replaced it. Once I'd done that, my board ran correctly.
I also don't know how this function went missing. I've had issues with a few lines of code being missing here and there since I started using EasyMod...I think I'm going to have to start doing my updates manually.
Before I came to this board to search for my particular error, I went through after I'd EasyModded my .21 update and checked each step individually...I found that, although this had nothing to do with the problem I was having, the final step in the update had NOT been done, even though EasyMOD reported that all files had been modified successfully.
Also it seems that EasyMOD for some reason inserts blank lines everywhere...many of my files look like they're quadruple-spaced. Not that this affects the functioning of phpbb, but still, it looks bad and when you're given a line number as a reference it's useless.