Got the same mixed-content error after changing my site to https://
The problem is caused by this lines lines
664: ‘https://{s}.tiles.wmflabs.org/osm/{z}/{x}/{y}.png’ => __(‘Wikimedia Labs OSM’, UMS_LANG_CODE),
665: ‘https://{s}.tiles.wmflabs.org/osm-no-labels/{z}/{x}/{y}.png’ => __(‘Wikimedia Labs OSM No Labels’, UMS_LANG_CODE),
666: ‘https://{s}.tiles.wmflabs.org/bw-mapnik/{z}/{x}/{y}.png’ => __(‘Black and White’, UMS_LANG_CODE),
667: ‘https://{s}.tiles.wmflabs.org/hillshading/{z}/{x}/{y}.png’ => __(‘Hill Shading’, UMS_LANG_CODE),
668: ‘https://{s}.tiles.wmflabs.org/hikebike/{z}/{x}/{y}.png’ => __(‘Hike and Bike’, UMS_LANG_CODE),
in wp-content\plugins\ultimate-maps-by-supsystic\modules\maps\views\maps.php
changing https:// to https:// solved for me the problem…