I wish you had come on here first and we would have told you that using any TK tool was not a good idea since they always seem to have issues and glitches.
Use this information and hopefully they will be able to get your website RSVP feature back up and running:
Contact Customer Service at feedback@theknot.com or call Mon-Thurs 6am-7pm PST and Fri 6am - 6pm PST at 877-335-5252.
Just to let those of you having issues know, tech is aware of the issue and they're actively working to try to fix it. I haven't received an update yet on when they believe it will be fixed but I'll try to keep everyone updated as I hear anything else.
So sorry about this but if you have anymore questions please let me know!
Re: RSVP Not working - Error message
You could also cross post this on the Tech Help board and tag @KnotRiley.
http://forums.theknot.com/categories/knottie-help
Good luck.
KR