Elgg1.9 Core plugin error after installation

Hi,I have installed Elgg1.9 on a directory of my site. The installation process was without problems, but if I check in the admin area the plugin I get a long list of:

This plugin is invalid: Missing plugin ID (guid 394)
Check the Elgg documentation for troubleshooting tips.
more info

Location: /web/htdocs/www.mysite.com/home/elgg-1.9/mod/

None of the core plugin works. I have tryed to upoad another plugin in the /mod directory, but with no result.
The guid number increase at any reload of the page. Don't know if it has any meaning.

The permission of dirs and files is 755, I tryed to change to 775, but no results too.

On the same site, in another dir, there is a working elgg 1.8.18 installation.

Any idea?

Thanks,
Teo