Could it be a case where different version of runtime library installed on the client machine (e.g. older service pack, or one that had been modified with software like ReFox) ? I'd seen mysterious problems like this on occasion when that occurred.
I'd also run into similar head-scratcher and discovered that the project included a different version of a class library than what I was expecting -- thus the EXE behaved differently from the code in the IDE. And in the case if I'd excluded the class library, then it was sometimes a toss-up on which library would get found first.