What is this mysterious shape/outline?
Hi everybody, I suspect my problem can be solved easily.
While I was working a mysterious square appeared in the place where previously a text box had been: see figure below. I seem to be able to select it: the outline becomes then slightly darker, but no traction points appear and it cannot be deleted by pressing "Delete". I would like to remove it.
It could be something obvious to you all: I have been using Illustrator for years, but I am not a designer and I always use only very basic functions (text boxes, arrows, coloured diagrams, placing external images). Do you recognize this square?
I am using Adobe Illustrator CC, release 2017.0.2, version 21.0.2 64-bit.
I guessed at a couple of tags that MIGHT have something to do with the question. Thanks in advance.

