I had exactly the same thing on IOS a couple of weeks ago. I had to clear my cookies, but as I use Chrome on IOS rather than Safari (IOS Safari is completely useless for forum posting) there doesn't appear to be a facility to selectively delete cookies, it's all or nothing.
--- a good idea, but 'data saver' is already off. Thanks.
BTW ... the direct link to 'replies To my posts' always works... only the 'show unread posts'.