Widget Logic and wp 4.3 Problem

I’m also experiencing problems after the 4.3 update. The widget is shown in the preview but as soon as I change the widget logic slightly (by deleting some whitespace) the preview gets recalculated and the widget is gone.

I read about the PHP4 Style constructors being a problem in some Widgets here but that is not the case with mine.

Any ideas what could cause this?

My widget logic is something like this:
is_page( array( 'page-slug1', 'page-slug2', 'page-slug3','page-slug4') )