Got an eerily similar problem (svg-support version 2.5.5):
Fatal error: Uncaught TypeError: Cannot access offset of type string on string in /wp-content/plugins/svg-support/svg-support.php:90
Stack trace:
0 /wp-settings.php(392): include_once()
1 /wp-config.php(132): require_once(‘…’)
2 /wp-load.php(50): require_once(‘…’)
3 /wp-admin/admin.php(34): require_once(‘…’)
4 /wp-admin/network/admin.php(13): require_once(‘…’)
5 /wp-admin/network/index.php(11): require_once(‘…’)
6 {main}
thrown in /wp-content/plugins/svg-support/svg-support.php on line 90
This is when I try to open the Network-Admin page of a multi-site installation.