SSL Mixed Content with Avia Timeline and Team Member Elements/ can't change URL?

Hello , i hope i can explain this properly! I really like the enfold theme!!
But since i am quite new to this field I could really appreciate some help!

We activated our SSL certificate and tried a lot to get rid of mixed content warnings in firefox. First it seemed to be somehow related to our AVIA Timeline elements ( with integrated images) so we got rid of these( deleted, replaced images but finally sadly just replaced these elements completely).
Seemed to be fixed but now the mixed content warnings point to the team member elements we have on our page such as these
https://greatl.de/team/ but also on all of our blogposts…

I was wondering and did some research… we already have installed several plugins: first really simple SSL” which we changed to ” WP Force SSL” .
and we tried to solve the issue with ” SSL Insecure Content Fixer”, and “better search replace” ,

nothing worked..

now I found that one should change the URL from http to https in the general settings as well.. but this does not work because the fields are greyed out??? what is the reason for that? and how to change it without messing up the site??

I would like to find a general solution as this appeares with each new blog post.. and would love to get rid of unneccessary plugins…

Plaese can you help me?