We are working on this case, and it will be solved in the next release. You can apply a simple workaround so you do not have to wait for a new release.
Connect to your instance via SSH and execute this command to configure the right permissions:
sudo chmod -R g+w /opt/bitnami/apps/openproject/htdocs/tmp/
After that, your instance should work without any problem. If, despite the workaround, the problems persist, please let us know.
Carlos R. Hernández