The zoomviewer tool creates IIIF manifests with http (not https) image URLs.
For example:
https://tools.wmflabs.org/zoomviewer/proxy.php?iiif=Califate_750.jpg/info.json
{ "@context" : "http://iiif.io/api/image/2/context.json", "@id" : "http://zoomviewer.toolforge.org/iipsrv.fcgi/?iiif=cache/1774f761854d805ec2e69fe35f683e63.tif", "protocol" : "http://iiif.io/api/image", "width" : 1492, "height" : 1248, "sizes" : [ { "width" : 186, "height" : 156 }, { "width" : 373, "height" : 312 }, { "width" : 746, "height" : 624 } ], "tiles" : [ { "width" : 256, "height" : 256, "scaleFactors" : [ 1, 2, 4, 8 ] } ], "profile" : [ "http://iiif.io/api/image/2/level1.json", { "formats" : [ "jpg" ], "qualities" : [ "native","color","gray" ], "supports" : ["regionByPct","sizeByForcedWh","sizeByWh","sizeAboveFull","rotationBy90s","mirroring","gray"] } ] }
The image URL here is http://zoomviewer.toolforge.org/iipsrv.fcgi/?iiif=cache/1774f761854d805ec2e69fe35f683e63.tif
Because the image URL is http, not https, it cannot be loaded from a secure context (e.g. an IIIF viewer served over https).