Error Message - Unexpected error encountered when opening report in Power BI Desktop

Mine is the same, so we could be onto a winner here.  I will sometimes get an error on tabs without the matrix visual too, but only in report files where i've used it.

I might make a test file where i remove this visual and see if it fixes it.

Nope that didn't fix it.  I've now gone to a tab, removed every visual and done a Save/Close.  Going to this tab has not spawned an error.  So i'll try removing visuals 1 at a time until i find it.

Are you by any chance using a custom visual "Hierarchy Slicer"?  My error seemed to go away on the tab where i removed that visual.