finding out what is causing dreaded "The website encountered an unexpected error. Please try again later." message

If dlog/watchdog and php error log aren't working, try this:

go on /includes/error.inc comment line line 245

print theme('maintenance_page', array('content' => t('The website encountered an unexpected error. Please try again later.')));

and put this

print theme('maintenance_page', array('content' => t('The website encountered an unexpected error. Please try again later.'.$fatal.' '.var_export($error,true))));

Very similar, you put var_export($error,true)) at end of error

By clearing the logs at /admin/reports/dblog then reloading the broken page I easily could identify the offending module when I rechecked the log.

Site is working fine now : )

You definitely should be disabling all contrib modules to perform the upgrade. Then enable them a few at a time after they have been replaced with the D7 version. When replacing the module, be sure to clean out the entire module directory, just in case some old files remain.

This should help narrow down the problem and be sure to check the php error log in case you find something there. (Although that is usually in the case of the White Screen Of Death.)