Notice: Undefined offset: 1 in og_membership_from_group_and_user_context() when trying to send an email to a group.

Hi there,
I´ve installed Privatemsg Groups, and I can´t send to each group any email.
I select the group, hit send, and it says that the message is sent, but the messages do no arrive.
At the error log I get this data:

Notice: Undefined offset: 1 in og_membership_from_group_and_user_context() (line 28 of /home/nets/public_html/sites/all/modules/og/plugins/relationships/og_membership_from_group_and_user.inc).

The only reference to this error, I´ve found it here, at the OG issue queue (it isn´t exactly the same though). And even when they say that getting the OG relationship shouldn´t be a problen in latest version (I assumed: 7.x-2.2), the problem is still there even with the recent version of OG.

Any thoughts?

Thanks!!!
Rosamunda