I changed the hosting of my B2Evo blog to SSL. The only necessary configuration change was the $baseurl in conf/_basic_config.php. But there remained an issue with mixed content. Images of posts were still rendered by http, because the image links are hard-coded in the database. The following query helped:

UPDATE evo_items__item 
set post_content = REPLACE(post_content, 'http://www.fabianfier.de', 
'https://www.fabianfier.de') 
where post_content LIKE '%http://www.fabianfier.de%';