I'm experiencing that bug too.
The problem is in the bonus prize during this promotion, which is automatically sent to the bank. Unfortunately, when heartments are the prize, I can't claim them because it claims I don't have at least 2 bank spaces free.
I have at least 10 f2p bank spaces free. I even tried freeing up members bank spaces by selling a few things. It didn't work. I can't continue with Treasure Hunter at all because one cannot proceed without claiming the prize. When both 'Claim' and 'Backpack' lead to the message, there's no way to proceed. No way to discard or convert to oddments since the seasonal oddments - heartments - are the prize, and there's the bonus prize that goes with it that you're not given the option to turn down.
My guess is that the extra bank spaces I purchased with Bank Boosters aren't being counted. But I'm just guessing. I don't know what's causing this glitch. But it prevents me from participating any further in the 'Gifts from the Heart' promotion.
13-Feb-2023 01:47:19