did you rename a collection maybe? (which may have moved the files)

Hmm, I thought each collection had a unique unchangable ID, like a number behind the scenes, from birth to death ?

It happened to me 1ce before but then I didnt find the photos, but restored them from a backup. Perhaps they were under a different collection, but I didnt look.

Can you tell me which name exactly is it I have to be careful about, one of these ?

Note: I might have changed some name, I dont remenber.

looks like it could be the URL "filename", default media folder it says, but still, how could it happen ?
Something like this:

• First call the original blog "blog"
• Then make another blog also called "blog"
• And the rename original blog
  (after which the problem would be there ?)

But why not every folder and file, half the empty folders are still there. ?

url filename is used to create a folder in your /media/blogs/ folder to store your images. If you rename the collection, b2evo tries to move the files. Normally it works... but maybe for you it failed. Please check your /media/blogs/ folder.