It's probably members that are in weak axis bending with the web in compression. They are probably violating a maximum b/t ratio for flanges (and the beam web would be considered a flange for weak axis bending) given in AISI.
What version of the program are you running? At one point, this error would occur when any minor weak axis moment put the web into compression. But, I thought we (RISA) had added some tolerance which bypassed this check for low levels of applied weak axis moment. I could be wrong about that, though. My memory on the issue isn't very clear. If you posted your model, I could tell for sure.