Uploading only parts of selected photos


I have started using Nextcloud on a raspberry pi3 b+ with Openmediavault installed
Linux omv 4.19.42-v7+ #1219 SMP Tue May 14 21:20:58 BST 2019 armv7l GNU/Linux
php7.0 installed

My problem is that when I try to upload multiple photos(for example 30) from my Android device only 10 or less are uploaded.
They don’t appear anywhere as failed. I don’t see any error message.

Please advise what to check if there somehow a way to open a debugger.