Hi Annette,
I use Moodle 3.1.1 on Bitnami in Windows 10. Sorry, I can't help you fixing a problem with Bitnami.
If you have material that you still need in your Moodle Bitnami 3.1.1, I recommend backing it up. Then uninstall Moodle Bitnami 3.1.1 completely. Download a fresh Moodle Bitnami 2.9.9-0 from HERE and install Moodle. Rename the htdocs folder to htdocs_2.9 and add a new htdocs folder. Copy the config.php file from htdocs_2.9 to htdocs.
Download Moodle 3.1.1 latest from HERE. I uploaded a zip-file. Copy the extracted zip-file to htdocs. Now you should have a functioning Moodle 3.1.1 on Bitnami.
With kind regards,
Hartmut