7.4. -> 7.10.5 upgrade: Fatal error: Uncaught Error: Call to undefined method EmailAddress::getSendConfirmOptInEmailActionLinkDefs()

What do you have in your config.php for these variables?

site_url
host_name

host_name:

insights-crm.de

Site_url:

http://insights-crm.de 

–> changed it to

https://insights-crm.de

I don’t know, this doens’t look like a SuiteCRM issue, it looks like a web server configuration issue, you just need to get your “www” redirects right…