We implement this fix in the version
2.1.3 was not released by Bitnami, so I supposed that you have upgraded manually your Magento from a lower version (affected by this issue) to
2.1.3, so the issue persists because it is not a new release of Bitnami.
If you can connect via SSH to the server, you can try this workaround to solve the issue:
$ sudo chmod 775 /opt/bitnami/apps/magento/htdocs
Carlos R. Hernández