"ERROR: File doesn't exist" when trying to transfer files to iPhone

Question

I am running Ubuntu 14.04 and want to transfer some videos to my iPhone 5 (iOS7) to it.

When I connect the phone it is mounted correctly and I can see all the DCIM, Photo... folders in nautilus. But where do I copy my .mp4 files?

I have searched the net but all is related to syncing music files (mp3). here, here, here.

Also, when I try to copy something in Document Folder or inside any app (2nd mounted iPhone point) it gives me an error:

ERROR: File doesn't exist

So, is there any folder in 1st mounted point (like DCIM, Photo etc..) where I can simply copy my MP4 files and watch it in my iPhone?

(NOTE: I don't need sync function but will not harm if it can be achieved).

EDIT:

Now I see two more problems:

1. For some reason my Document Folder is not mounted any more (even after unmounting and restarting). Only the Picture, DCIM etc. folders are mounted.
2. I sometimes get this:

EDIT 2: When I tried the troubleshoot method in 1st link of duplicate I get Error: Pairing with device xxxxx failed with unhandled error code -20. (And my phone was not locked)

Also, I get on re-connecting:

Answer 1

The answer here should help here.

To summarise it:

1. Install a video player app on the device such as "Flexplayer".

2. Plug the device into your computer via USB

3. Navigate to "Documents on My iPhone" in your file browser. If you do not see this under mounted devices, make sure the libimobiledevice-utils and ifuse packages are installed on Ubuntu.

4. Find the folder for the video player you installed, eg "Flexplayer".

5. Drag the video files into this folder.

6. You may need to close and re-open the video player app on the device before the new videos appear.

Note that iPhones and iPads do not act like regular drives on your computer where you can access all files as if it's a USB drive or MTP device - if you try this all you'll be able to do is see photos taken by the device camera. Instead you need special drivers such as those used by iTunes. However, Ubuntu now has built-in support at least for the ability to access document storage on iDevices, as described above, and that's what provides the "Documents on my iPhone" mount point.