"You must log into this network" - What network?

Would you be able to attach a screenshot of that dialog to help understand where it's coming from?

Have you managed to work around that? For example using a new Private Window to sign in, or just restarting everything, incl. your router/modem etc.?

Thanks for the response. I had other things going wrong suddenly as well, so I did a full system restore to a backup I had done in May and all the problems have disappeared, including this one. I guess I had been hacked without my knowing it at the time.