OK.. a quick update on partial solution...
On AGOL, I did a bit of digging around and our site's default basemap was set to 'Topographic'.. but a version of this that was based on the deprecated raster format. So I switched the default basemap over to the vector version of 'Topographic' (also tried other vector basemaps) and now the report with the map element included works OK... at least on AGOL
On our Enterprise environment, I created a webmap that had the same content as the vector version of Topographic I'd used on AGOL. I set the sharing permissions on the webmap to Everyone. I configured the map report template to include the following line..
${T98GrilleInspection_point | map: “c8c2f89361284afdbdeb48bd7889fbc6” | size:300:200}
but when I try running the report, I'm still getting the 'Failed to print map... ' message - see attached screenshot'..
Any suggestions as to what the issue might be?