I was unable to upload any photo or video after enabling backup in the Google Photos Android app. It always stuck at “Getting ready to back up”, no matter how long I waited (10+ hours), cleared caches/data or reinstalled.
First I thought It was media reencoding but actually it was not. After reading through the Google forums, I found this post.
The problem is close to the media indexing service, probably related to issues indexing some mkv, flac or other formats.
So the workaround is to disable access to the non camera folders by creating “.nomedia” file inside all folders containing possibly problematic files. The side effect of this is that the media can no longer be visible to other apps (some apps can be set to ignore the “.nomedia” option).