There may be an issue in the STACBrowser with thumbnail URL encoding.
The STACBrowser seems to “encode” the URL (provided by DLR) causing a broken link in STACBrowser. Pasting the original URL (present in the STAC GeoJSON representation) in the Web browser allows to see the thumbnail…
The URL encoding comes from urijs and can't be disabled without major effort. The other thing that confuses me is that Firefox shows an equal sign at the end for the requested image...
Reported by @ycespb