These aren't errors but warnings, and they generally aren't serious. You, or your host, should adjust the php error reporting settings to make these go away.

Lluís wrote:

I have created a site and subsites sharing some tables, and when a user with uid!=1 tries to create a node, on top appear a red message with: warning: Invalid argument supplied for foreach() in /home/escolcat/agrupaments/includes/form.inc on line 950.

Despite this errors nodes are created (don't know if correctly).

How can I track this error? I know with the provided information is impossible to solve it. I also use Organic Groups, cant it be because a contrib module?

thanks