What’s your .htaccess look like? Is it perhaps redirecting in a loop until it hits the threshold and gives up? The apache error log will likely have some warning on this. I’d start there first.

The error log shows unfortunately nothing and the .htaccess only contains the standard wordpress entry.
However I have a suspicion that the problem is related to the WP Super Cache plugin.
The small droplet runs a relatively heavy wordpress site. For some reason the cached static content is not loaded when the naked domain is accessed, instead the dynamic PHP content is called for every request.
For now the problem is solved with following .htaccess rule:

RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^(.*)$ https://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]