Acrobat won't sign the PDF

Hello,

For some reason, Acrobat won’t sign the PDF.

Once I draw a rectangle, I get this massage: “This signature field requires a digital signature identity. Would you like to configure one now?” and pressing “Configure Digital ID” fill the whole document with 50% transparent black with a white pixel in the middle.
Afterward, only Esc will make the document available again (without the signature).

I’m working on an up to date Acrobat CC and I tried “Save as other > Reader extended PDF”.
What should I do?

Many thanks,

Amit