All OS Open xyz (UK Ordnance Survey) base maps show as a fuzzy image background
If that is a WMTS service underneath, then it's not currently supported. We're planning to support WMTS within the next couple of updates.
Not sure if OS Open Data is WMTS, Esri(UK) might know, but that is most likely the case.
We are seeing the same sort of thing on this end. It won't display the default base map or our tiled base maps. Just a fuzzing background as you've shown above. GE Pro works just fine with the same data.
And you are pointed at Esri RESTful services? Can you please give me some more details on Portal version, etc? Do you have any public links exposed we could point at?
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