I think this is one of the simplest ways to check the Id to your "Global Library"

*Go to Admin Mode
*Click "Set Access Rights"
*You will see a Page Tree that contains the "Pages" representing your Global Block Library.
*Check what href the "Global Library" node in the tree have. For example in a new EPiServer site it will be "3" but if you have upgraded it will certainly be a much higher value.

Thank you so much, that did the trick :-)

I had a similar problem which could not be solved from the information above, but since I found a solution I'm posting it anyway in order to maybe help someone else =)

I upgraded an enterprise site but failed to migrate the VPP files to media content. The migrator failed with the message "The content with id x could not be found". Debugging the migrator, i found that it tries to get the children of 

a) The global assets folder (same for both sites in my solution)

b) The site specific assets folder (distinct for both sites)

It was the site specific asset folders that failed. They did have content ID:s, but the content ID:s did not have any corresponding content saved.

The solution was pretty simple. I went into admin mode and website management settings. For each of the sites, i unticked the "Use site specific assets" checkbox and saved. The i ticked them again and saved again. In the log, I could see that new content was saved to the content repository when I did this, with content ID:s that differed from those that the migrator tool failed to get. Then I ran the migrator again and saw that the content id:s for the site specific folder had now changed, reflecting the ID:s of the new content that was saved when I re-ticked the checkboxes. Now, the migration worked!

TL;DR; Go to admin mode, untick the "Use site specific assets" checkbox and save for all sites in the solution. Go back in and re-tick the box for all sites. Re-try migration.