Gallery does not work in combination with External Memory (Dropbox etc.)

Some people here wrote that the Gallery does not work.
I had the same problem!
If you use a connection to an external cloud, the gallery does not work anymore.

If you remove external cloud from Nextcloud, the gallery will work fine!

I think there is an update needed for Gallery!

I think previews of files need to be enabled on external storages to have support for Gallery.