Bad link to /user/perm_login?status on every visit to site

I updated from 2.8.1 to 2.8.4 a few weeks ago and also updated a few downlevel plug-ins on my site at the same time. Since then, however, every visit to my photoblog (https://www.davewilsonphotography.com) is resulting in a warning email from WordPress telling me that the target page (basically any valid URL in my site) generated a 404 on an attempt to access https://davewilsonphotography.com/user/perm_login?status.

I’ve downloaded a copy of my site and grepped the tree but no files contain the string “perm_login”. Can anyone offer any hints as to what is causing this and how I can get rid of it? My inbox is being absolutely flooded with these messages. I have yet to try disabling each of the plugins to see if one of them is causing the problem but will give this a try over the weekend.

Thanks in advance for any help,

Dave Wilson