Non SSL content is not allowed within SSL pages. For example, HTTP image, HTTP iframe etc (exclude HTTP link). Check browser warning with following procedure.
In the case of static content, convert data in the database from http to https.
In the case of dynamic content, you may need to modify PHP program.
I suspect it's due to the embed plugin, here is a fix posted in github
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April 20, 2021: Q2A 1.8.6 released!