I just fixed the following problem in our installation: https://forum.omeka.org/t/omeka-s-encountered-an-error-add-an-item/5134
It took me quite some time, to figure out, what the problem was, since the message I got was just “Omeka encountered an error.”
To me looks like adding media to items will never work without having ImageMagick installed.
I would therefore like to suggest to change the following paragraph of the installation instructions:
“Optional, to create thumbnails: ImageMagick version 6.7.5 or greater, the PHP imagick extension, or the PHP gd extension.”
Thanks a lot for the wonderful software you provide !