The \rule behaves well in every case and doesn't reproduce the error. So I'm not able to provide you with an MWE.
The problem occurs with the \lhead statement as well if I put the \picture statement in the square brackets istead of the curly brackets of the \lhead statement.
I'm trying to get images in the header. I succeeded in putting one on the left header side of the right page. But I'm unable to put it on the right header side of the left page.
I'm using fancyheaders and the working code for the right page is: