Can you try?
in \plugins\fabrik_element\user\user.php
change line 189
$layoutData->value = is_a($user, 'User') ? $user->get('id') : '';
to
$layoutData->value = is_a($user, 'User') ? $user->get('id') : 0;
Thanks! BTW - how do I update to the most recent files (so I make sure that I am not reporting something that has already been fixed?
I see this here (however, I don't see a url): https://github.com/joomlahenk/fabrik/wiki/Testing-a-fix: Alternatively, you can install the cumulative update zip from our download server. A link to this file will be included in the changelog wiki.