I had the same issue (HTTP 500 error, unable to pay with points in app/on Amazon) when my TYP account was locked by Citi. The error goes away when my TYP account was unlocked.
I just tried and thankyou.com is working for me. I would call again and see if you can redeem/transfer points by phone. Hopefully it's really just a website glitch (it's Citi after all) for your account