A duplicated class (caused from just copying a block from a page to the header) was the cause. I was aware that same classes on same page interferes, but not that also same classes on headers causes issues (but logically it is clear). Is there a shortcut for copying blocks while automatically change the core class of a block? I currently always have to duplicate a block at the source (as this creates new id’s and classes), then copying it to the target, and then deleting the duplicated block