When I've run into this issue, it's either a permission issue for the user (no access to an entity), or the microflow call is coded incorrectly in the widget. So, check permissions first, and if that's not it, share your code here and we can help you troubleshoot.
It's not easy to pinpoint your issue based on what you describe. There are a few things that I can think of:
If you have your widget code on Github (or share it on Github as a Gist), I am happy to have a look at it.