Usually it's due to a typo or something unusual on the prior line--i.e., if it says "line 45", look at line 44 also. Usually what it means is that something is messing with the usual input of the session file. It could occur on the same line where DirectRT gets confused--but again, it could occur on the previous line--e.g., if the previous trial has a stimulus with a location but no time value. Or if there is a missing WGR, or BGR column--that kind of thing.