Sometimes the Mapillary web app enters a broken render state (see screenshot below): the panorama is distorted into irregular polygons and partly disappears. When this happens, the URL in the location bar always shows x=NaN&y=NaN instead of the usual small floating-point values. Removing the x and y parameters and reloading always fixes the problem.
A similar problem was reported for Mapillary imagery embedded in the iD editor here: Mapillary Vorschaubilder-Phänomen - Deutschland (Germany) - OpenStreetMap Community Forum . The context is different (embedded in iD), but it might use the same underlying viewer or rendering code.
