wp-admin mixed content when loaded in SSL

My wp-admin is forced to SSL. I get the following mixed content error:

Mixed Content: The page at 'https://cdn.wpdownloadmanager.com/notice.php?wpdmvarsion=2.8.95&origin=https://www.culturedfoodlife.com/wp-admin/' was loaded over HTTPS, but requested an insecure image 'http://cdn.wpdownloadmanager.com/wp-content/uploads/thenext-thumb-cache/all-themes-637a0a11cac0c54e29291168596988bd-900x0.png'. This content should also be served over HTTPS.

https://ww.wp.xz.cn/plugins/download-manager/