Strange background in my Gimp

[Krikor]

Hello everyone!

Just today I noticed this "spot" on my gimp screen.
A faint, barely perceptible image, like a watermark.
I've already restarted the Gimp, and that background image is still there.
I tried to give a screenshoot, but then it doesn't register in the generated image.
I needed to take a picture of the screen to be able to show this stain.

I think I can read "INSCREVER-SE" (Sign up, in english) as part of the image.

How did this image come about?
How can I remove it?

Thank you for your help!