[support] foreach error on forms with non-admin user

[Lluís]

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

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