How do I troubleshoot 'Theme key "" not found'?
This warning message comes from theme() function in includes/theme.inc. For temporary debug you could just add ddebug_backtrace() there (if you're using Devel module) to see where the call comes from (and see what are the params). If not using Devel, just check output of debug_backrace(). And then it all depends what you are going to find out...