Never had a problem cross platform if the AE versions match, the plug-ins and scripts are installed on all machines, the language is the same and the footage is compatible with both windows and Mac. As long as you are all using the same language for your AE version expressions will work fine. There are no methods that are unique to one OS.
Thanks Rick, we will reconfirm our settings and respond back.
Rick, I just confirmed that all versions of AE and plug-ins on the render farm and the designers stations match and the job still fails. In an effort to get around the expressions issue our designer is redesigning the "master project" using scripts and not expressions to control the individual layers. The thought behind using scripts is that the script is an on/off function (it adjusts a layer and turns off) and will not be sent to the render farm. The expressions were embedded in the project (remained in an "on" state) and were trying to be processed by the render farm. The projects that have been redesigned using scripts have been rendering successfully so far but I am interested in investigating why the expressions failed now that you say there should be no cross platform issues.