I get an error “Invalid WidgetID”, is it a bug?

No, this is not a bug. Did you hit the Save button after adding the new widget and then going to the Dynamic Widgets options page? When you did and still receive this error, you’re probably using a widget plugin. Other widgets work fine. Dynamic Widgets can’t do anything about it. The widget plugin is using an old and deprecated way of adding a widget to the WordPress system. I recommend you to contact the author of the plugin to update it and point the author for directions to this blogpost from 2009.

Like Dynamic Widgets? Like my FB!

All about QURL