Is it possible that you accidentally closed the request, rather than accidentally rejecting it? If so then this situation is probably related to an issue where group invitations and tournament invitations can’t be resent if they have been closed with no response. See also: Group join request notification hidden forever - #12 by Kosh
If that is the case for one of you, it is still possible for the other to send a request in the other direction. If the problem has occurred in both directions then I fear I have no work-around. I tested on Beta to see if blocking would cancel the old requests and allow fresh requests after unblocking but unfortunately not. Work-around ideas anyone?