Quite possibly a corrupt control, or the link between it and the code project is broken, or the property for the event is still in the property sheet and the event was deleted, or the code project needs to be compiled, or there is something amiss about your form code, or ...
The error message is about not being able to find the specified event, not losing focus?
EDIT
Also, more errors are encountered when any action attempts to loose that same focus.
I'd say that all of those actions are trying to set the focus to the same thing, hence you get the same result.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.