Deleting the fta_settings did not help. The error log of the web server shows the following:
[Tue Oct 08 10:49:49.441514 2019] [php7:warn] [pid 66542] [client 193.81.143.52:47078] PHP Warning: key() expects parameter 1 to be array, null given in /var/www/apache24/wordpress/wp-content/plugins/easy-facebook-likebox/my-instagram-feed/includes/mif-skins.php on line 99, referer: https://www.obermassing.at/wp-admin/plugins.php
[Tue Oct 08 10:49:52.160175 2019] [php7:warn] [pid 66542] [client 193.81.143.52:47078] PHP Warning: key() expects parameter 1 to be array, null given in /var/www/apache24/wordpress/wp-content/plugins/easy-facebook-likebox/my-instagram-feed/includes/mif-skins.php on line 99, referer: https://www.obermassing.at/wp-admin/plugins.php?activate=true&plugin_status=all&paged=1&s=
[Tue Oct 08 10:50:00.452549 2019] [php7:warn] [pid 30434] [client 2a0c:ec80:55:161::6:59820] PHP Warning: key() expects parameter 1 to be array, null given in /var/www/apache24/wordpress/wp-content/plugins/easy-facebook-likebox/my-instagram-feed/includes/mif-skins.php on line 99
Interestingly, if I activate the plugin it somehow creates pages with the title “My Instagram Feed Demo”.
I have to delete them after activating the plugin.
I run a multisite installation and it is the “Graphene” theme. On other blogs it works, hence, it definitely seems to be theme-related.
Bernhard