simplenews causes failure in administer->content

I'm completely puzzled about this. Didn't find anything in other postings:

When I activate simplenews module. The filter button in the administer-> content list page (Show only items where status= etc.) stops working. When I set a filter and press the filter button the same list (with unfiltered entries) appears and the filter has no effect.

I traced the problem and found this:

1) the $_SESSION loses it's content and therefore the filter does not work. ( the filter remembers the filtering in $_SESSION to allow mulitple filters to be added (refine filter).

2) I traced the activity which happens in simplenews when the administer content filter is activated and found that the line ~68 "drupal_set_html_head(theme('stylesheet_import', $stylesheet));" in function function simplenews_menu($may_cache) of simplenews.module causes the problem. If I remark this line the filter of administer content works fine.

3) I thought it may be a memory problem so I added ini_set ("memory_limit","20M"); to index.php. No change. ( how can I check my memory usage and allowed memory ?)

Does anyone have a clue about fixing this ?

Thanks
Nbd