It sound like a problem with the left side of an equation. I'm not sure what the source is, but I've seen this type of error in relations before. It happens when I try to incorrectly assign a string to a numeric parameter or something like that.
I'm just guessing, but I'd say AF0 stands for Assembly Feature number 0. This may be in response to an issue in feature level relations, Pro/PROGRAM, or feature level parameters.