I have found a problem on Ultimate Branding plugin. It's not compatible with Multinetwork ("Login Image" and "Custom Multisite Favicons" modules).
My proposed fix: replace the 3 occurrences of
$mainblogid = wp_get_sites(); switch_to_blog( $mainblogid['blog_id'] );
2 occurrences in ultimate-branding-files/modules/login-image.php
1 ocurrence in ultimate-branding-files/modules/favicons.php
obs: this is a partial fix. It will only work when the id of a main blog of a given network is the lowest id between all the blogs in that network. Works for me but will still fail otherwise. I am not sure how to obtain the main blog id of a given network by another method.