Hello,
there are no real consequences to this bug, just a low priority display issue to resolve.

Press F9 and visit your blocked users list/block a user/click on more to enter a username.
As soon as the input dialog to enter the username opens, the following is also added to your history:

Unknown player: -3

As I said, this doesn't really cause any major problems, entering a username to block still works perfectly fine, it may only be a little confusing when looking back at the history.

Thanks.

Replicated.