Could you confirm that the Wordpress Multisite you are using is one of the latest available? In any case, could you try entering the machine via SSH and execute the following commands?
sudo chown -R bitnami:daemon /opt/bitnami/apps/wordpress/htdocs/wp-content
sudo chmod -R g+w /opt/bitnami/apps/wordpress/htdocs/wp-content
Hope it helps.
Javier J. Salmerón
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