Discussing and posting

[Stuart McLachlan]

There are now several free/cheap  SSL sources.

I know. But it needs to fit into my other (SSL) domain. AFAIK Apache accepts only one (combined) .crt file. OTOH MyBB needs to allow a transition of all internal links from http to https. E.g. Wordpress has its tools for doing so, but I don't know about MyBB.

You can get a certificate that covers richheimer.de  and pump.richheimer.de. It's common practice. (you could even get on to cover richheimer2.de at the same time)

You just need a .htaccess file with an appropriate REWRITE rule to get Apache to rewrite http to https. All it needs to contain is:
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]