I think that error message may be bogus coming back from SQL and I can't imagine what relationship it'd have with Exchange cleanup. That particular routine is doing a read only query on the vw_Subscriber view and finding any hotel guests that have been checkedout for longer than the number of days set for room archive time (3 days by default). For each of those it finds it calls the DeleteGuest routine.
No one should be updating the table schema other than on Unity upgrades so that error message is a tad puzzling. Is there something else going on that would cause the SQL server to be unhappy during this time frame?
Either way, turn on debug output in the Bellhop admin interface and send us the output file it creates containing one or more of these errors to look at.
Also, you can prevent Bellhop from doing these archived room checks during the time you have your Exchange server configured for cleanup on the Bellhop admin page. By default it's set to sweep between 1am and 4am - you can push it back to, say, 2am to make sure it avoids these maintenance windows that appears to be conflicting (though I can't say why Exchange cleanup would have any impact at all here).
