I designed and manage a kirby site that has been working great for a couple years. The site is a fairly simple portfolio-style site, every month or so my client will log in and add a project or change some wording. Recently they reported being unable to log in to the panel. Checking the error logs returned this:-
[19-Mar-2019 06:09:38 UTC] PHP Warning: Version warning: Imagick was compiled against Image Magick version 1650 but version 1657 is loaded. Imagick will run but may behave surprisingly in Unknown on line 0
(first instance)
I took this up with the host, they updated php which seems to fix the error but I am still unable to log in to the panel. The host says no maintenance has been performed this year. Turning on debug mode returns no errors. My client says they have made no changes to the site recently. The site itself is fairly vanilla kirby, but I do recall tweaking the thumbs settings and adding a simpleimage plugin as my client was reporting issues with uploading large images, but these have been working fine for a while now.
Try removing the .logins file from the site/accounts folder. Updating to a newer version would make sense, but don’t do that on the live site without prior testing locally.