Checked the database,
In wp_options we got:
siteurl -> https://www.domain.com/blog
home -> https://www.domain.com/blog
Changing both of them to https://www.domain.com and moving all the files back works fine, changing both of them to https://www.domain.com/blog/ (the extra / at the end) doesnt change a thing, too. (just if that would have been important …)
The wp-config.php is fine, too – the access to the database works I guess otherwise it wouldn’t work when I move them back, right?
Any other ideas maybe? I really have no clue what’s wrong here since it worked in the past more than once. :/