The reason is known now: The Icegram Popup Plugin the setting “Lazy load” caused the problem.
But a general solution I do not know, because of “lazy load” seams to be necessary to bypass Cache (we use W3 Total Cache) e.g. for correct Geo IP identification …
Sorry – as I wrote it works only for one time saving this post.
If you are doing (later) something else in this post and you are saving next time the “>” demages the post …