We had an issue earlier with the database and people couldn’t mark or unmark things as watched, so your issue, seems related to it. That has been fixed a few minutes ago, but it takes a while for everything to get back to normal.
Please, wait a few minutes and try again. If it still doesn’t work, go to your settings and ‘Reset Browser Data’. Then try again.
You can also try marking something as watched and unmarking it later, that also resets your cache.