Thank you for bearing with us while we try to debug this. We appreciate your patience!
It might be an issue from a plugin/theme. Is it possible to install this plugin: https://www.ads-software.com/plugins/health-check/ ?
After you install and activate the above plugin it has a Troubleshooting mode which allows you to have a vanilla WordPress session, where all plugins are disabled, and a default theme is used, but only for your user – your visitors will still see the normal website.
Go to its troubleshoot mode and only activate this plugin and see if it works to add a new gallery with a default WordPress theme (Twenty Nineteen for example) and no other plugins.
Please let me know how it goes!
Thank you,
Mihaela