What error did you see? Is this a problem in the browser, or in the phone?
If you had not logged in a few days, the reason might be because all of the authentication cookies were invalidated when I updated the lemmy version to 0.18.2. There was an exploit that resulted in these cookies from other instances being stolen. There is no indication that anyone in this instance was affected, but as a precautionary measure I invalidated the open sessions when I updated the instance. One possibility is that your browser still has the old auth cookie saved. If this is the reason, then you can manually delete the cookie, or go to the lock sign next to the URL and click on "Clear cookies and site data".
But if you had been logged in recently then this is not the reason, and I need to investigate further.