Thank you for your reply! I am playing the browser version on Google Chrome.
Regarding the Mystery Box freeze, it seems to be resolved on my end now.
Currently, when I click (purchase) a Mystery Box:
A message "OPENING..." appears for a few seconds.
Then, a pop-up window appears to show the item.
I can click the "Close" button and continue playing without any issues.
Regarding the freeze, previously, when I clicked the Mystery Box, nothing (like a pop-up) would appear on the screen and the game would freeze, but now it is working normally. Thank you for your help!