Menu doesn't show in one single slide

Hi dear community!

I have a problem with a larger Storyline-course: the menu on the left side doesn't show just in one specific slide. All the other slides - the menu shows in the player. And in the player-settings the menu is (of course) "on". But that one slide - once I preview it - nope. No menu. And I have no idea, why that is!...

I think it might be caused by the fact that I published just this one slide without player to review (to the incorporate it into a Rise-course). But that shouldn't affect the Storyline-course, should it? I also saved the whole file under a new name - but that didn't help either...
So, I would be grateful for some advice! Thanks in advance!