paradonym

For the people googling around and resulting in this page:

steps I’ve done:
-reinstalling WordPress via SFTP (using another table_prefix in wp_config.conf)
-recovering my posts from a previous data-export via tools -> data export (import via tools -> data import)
-increasing the php.ini memory_limit value to 256M (even if 64M should work too)
-cleaning the servers .htaccess of all stuff not inside # BEGIN WordPress and # END WordPress

Hope this will help you even if I can’t say which action was it to make it work 2 days later…

I manually reinstalled WordPress by deleting the SQL, deleting the WordPress files via SFTP and then re-uploading the default wordpress files with a new configured wp_config.php – so everything should be set to default settings and default plugins.
If I upload an XML backup of all the posts I’ve written before the error appears again. So I think plugins and settings problems are already checked as the error appears in a completely stock WordPress environment only having a recovery of the blogposts…

For my previous tries (personal data has been hidden):
“[Mon Jul 11 14:20:57 2016] [alert] [client $CLIENT] /var/www/virtual/$USER/html/.htaccess: Option All not allowed here, referer: https://$DOMAIN/wp-admin/plugin-install.php?tab=search&s=broken+link”

The .htaccess only consists of WPs default entries and an addition to redirect every http call to https. and some redirection rules to redirect some other permalink structures to /%postname%

All of the sudden, it somehow works now – without ever doing something additional… Strange…

Thanks for the help. Worked like a charm.

Google Authenticator Data isn’t grabbed by ADB (android developer tool) if you create and restore backups of the android-apps, so I got into this problem that way.