The problem seems similar to the behavior you might be seeing if you have outdated widgets (although the lack of logs in firebug is odd). The broken widget doesn't necessarily have to be on the home page as the client downloads all custom widgets on first access, regardless of where they're used in your application.
You could try running Tools > Check widgets from the modeler. If that also doesn't tell you what's wrong I'm afraid you're left with deleting custom widgets until your app runs.
Microflow label doesn't work in MX6 because it's not relevant anymore. In Mx5 a replacement has been introduced, you can use the action trigger (native modeler widget) instead.