Issue with large flashing black square.

Hello,

My problem is quite difficult to explain. Here it is:

When I want to write something, for example a web address, the character is preceded not by a small vertical line but by a big blinking black square.

Do you see what I mean?

In fact, it's really strange, maybe it's related to the screen resolution?

I’m not sure if anyone has the solution, so please let me know what to do!

Thank you in advance to everyone who reads this...

Rania