Nextcloud claims it's logged out every time W10 starts

Every time W10 boots, a Nextcloud window pops up saying “You have been logged out of … Please login again. Please switch to your browser to proceed”
Meanwhile, it opens a browser with 2 tabs:
One says “Access forbidden. Your login token is invalid or expired”
The other gives me a Login button to press which then takes me to a ‘Grant Access’ button and if I then press that, all is well - until the next reboot. At no time does it actually ask for a login or password.
I only access NextCloud via file Explorer and that continues to work all the time regardless of all this.
Nextcloud has been installed for a year or so and this only started happening a week or two ago.
I never access Nextcloud via the browser.

Any idea what’s going on?