I tried something out and got one of my chars stuck like that as well 
What I did was the following:
1) I sent an invite to one of my chars
2) I clicked the Message until I got to the Accept or Decline-screen
3) I deleted the House Invite Message from my Inbox
After this I can't invite my character anymore since it's said to have a pending invite already, just as Chronym said.
PS: Might need help to get the char I tried it on working again :p
Edit: Went back some pages and found the invite again. I don't know if it's account/invite-specific, but you could try the URL I had ( https://nordinvasion.com/house.php?accept=2574 )
What I did was the following:
1) I sent an invite to one of my chars
2) I clicked the Message until I got to the Accept or Decline-screen
3) I deleted the House Invite Message from my Inbox
After this I can't invite my character anymore since it's said to have a pending invite already, just as Chronym said.
PS: Might need help to get the char I tried it on working again :p
Edit: Went back some pages and found the invite again. I don't know if it's account/invite-specific, but you could try the URL I had ( https://nordinvasion.com/house.php?accept=2574 )