Backend corrupted after malware attack

Hi!

The website I manage has recently been attacked through the malware IOptimize. For about a week now (first occurrence seems to have been October 13th), all kinds of corrupted files have been uploaded to my webspace. The website itself has always been perfectly fine and is still usable. After I got alerted by the webhoster, I first changed all the passwords. But the suspicious uploads continued and today I was not able to use the admin dashboard anymore (basically, all styling is lost and all of the links lead to Error 403 Forbidden). Now I have deleted all uploaded files I could find and tried to reset htaccess and index.php. However, the problem persists: the website works fine, but I cannot edit it anymore or access anything in the admin dashboard. For the worst case, I do have a backup (five months old, but that would not be tragic), but what else can I do to get the backend going again?

The page I need help with: [log in to see the link]