When you set customErrors to Off, it showed you this error in the page right?
Error: Sys.WebForms.PageRequestManagerServerErrorException: An unknown error occurred while processing the request on the server. The status code returned from the server was: 500
Source File: http://mySite.org/ScriptResource.axd?d=l53QigJd3KGiEVv-3KumAbiX189-7RprPkX3_4MKfr48x-zQ4acbiUL9qUInbHCWu2s-MF7AB9KFkBJsbx9-r2_qlYOMuy2xsfFaHgwe6Hs1&t=68b57ea0
So thats what it is supposed to do if an error occurs and customErrors is set to Off. Sometimes you don't always get real helpful error messages depending on the nature of the error. Any unhandled error will be logged int he mojoportal log so that is another place to look for error messages.
Hope it helps,