Hello,
I just ended my holidays and have used the upload to cloud feature of the android client. Every time I had WLAN access, it started immediatly pumping the files. Great!
Now, I have many many photos and would like to present it on my tv screen.
Is there any chance to access the nextcloud gallery with kodi or the fire tv? What possibilities do I have?
Pity that my holidays are over 
Many greetings!
Making me jealous just thinking of holiday (had mine in March)
Depending how you store your NC files, you could create a source point towards NC. As taken from the Kodi wiki
When entering a source path through the GUI, be sure to include the dav: to start the address.
dav://username:password@server/share/
(source: http://kodi.wiki/view/webDAV)
You should then be able to view your photo’s on the big screen.
If these include a browser, you could simply load the Gallery URL and use the functionalities provided by the app itself.
Using DAV(s) is really an easy way to access it from kodi!
I created a kodi-user and shared read only the folder with it. Was a little bit tricky to get the webaddress, username and password (and remote.php/webdav/) typed in via the remote control, but it worked.
Unfortunately, as the photos got uploaded instantly, and the kodi-implementation of DAV is not very sophisticated, it now tries to scan about 3000 files 
Is there a chance to sort the files into folders later within the nextcloud-server by date, hour whatever?
The Android app for NC can do that at moment the photo’s are captured. For existing photo’s already on NC you will need to create a script that would generate the folders and move the photo’s. NC won’t do it on its own.
It would be best to first clean the NC folder, before importing it into Kodi. I am currently in process of moving from QNAP to NC and still working on how the setup should be and optimising folder and sharing structure. I know it will take me some time, but once that is done I can do the actual switch over with least issues.