What kind of SSL options have you got enabled at Cloudflare SSL/TLS tab for your domain?
“Full”. I have Let’s Encrypt at my webhost, but I guess that’s not relevant anymore?
Regarding .htaccess file and rewrites, kindly re-check if it is writeable file (CHMOD 644)
Yes, it’s 664. Read and Right for “Owner” and Read for “Group” and “Public” (translated back from Swedish, so the terms might no be 100% accurate).
What kind of WordPress permalink URL structure have you got selected in WordPress settings?
Custom structure: /blog/%postname%/
Maybe I should mention that I use WP Rocket and connected Cloudflare via that plugin. However, disabling WP Rocket (or Wordfence) does not remove the error message.
Meaning, the WPML should figure it out to make /blog/en/ or /blog/de/ or some other language, right?
I assume WPML thinks your /blog/ is a sub-directory of WordPress installation, which could be a reason why you cannot have /en/, /de/, etc. as “language directory”.
Either, this is on your main domain divimundo.com, while I beleive you have a valid reason why are you using it under the /blog/.
From other point of view, I do not understand why does WPML not recommend in that case, going by divimundo.com/blog/en/ or divimundo.com/blog/de/ regarding URL permalink scheme?
The best case scenario from my perspective would be to write a ticket on WPML forums and ask why you cannot have the option directory for default language if you have a permalink structure like /blog/.
Regarding WP Rocket, kindly see below articles how to configure it propperly, just in case:
I think the rewrite rule should go within /blog/index.php to make sure the /blog/en/, /blog/de/ would work, if the WordPress is installed under /blog/, or if it is installed under your root domain, but /blog/ is just a part of it to for example show the news and blog posts, so, I am not 100% sure again why WPML cannot work, but hm …