Files are invisible in the backend

In a MODX Revo 2.2.6 installation, I can't see or find any files in the backend. I can select media sources but I can't seen any files in them.
When I select the File Browser (the popout option, I'm using a Dutch version so I don't know the exact translation) I can see the folders and subfolders, but no files are visible. They do show up in the front-end, but there's no way to select them for TV or similar.

In the same problem, other installations (mostly 2.2.5) have been giving me problems with the thumbnails for certain images. Is it possible that the backend has problems creating thumbnails for images that are too large or have too large a resolution?

Go to System Settings and under Core > Lexicon and Language switch "Use Multibyte Extention" to No. If that fixes it, the problem was because your system doesn't have the PHP mbstring extension. It's fairly uncommon for that to be missing, but you do come across it once in awhile.

This definitely worked for the first problem, thanks.

Oh, about your second question. If your server doesn't have 'convert' (part of http://www.imagemagick.org/) installed, phpThumb will use the GD library for resizing. GD isn't very efficient with its memory usage and can't process large images. That could be the reason. Also non-alphanumeric characters in filenames can cause thumbnail problems, though I think this is fixed in 2.2.7.

(Occasionally a server will have ImageMagick installed, but phpThumb for some reason doesn't identify it and so still uses GD. It's a possibility.)