I tried it myself and saw the same problem:
After entering the password to the share, the gallery page
and the first couple of images would load.
The rest of the images were not visible.
When reloading the page, I had to provide the password again.
I upgraded to Nextcloud 15 in the hope that it would solve the error,
but it did not.
I only got into more trouble and had to
manually fix IMAP login
The gallery index page loads
All other images return a HTTP 401 status code and the following JSON:
When using a private browser window, it worked.
It also began to work when deleting all cookies via the browser developer
thanks you RSS link