Why does my Windows explorer incorrectly display available space of a mapped Koofr drive?
This is a known Windows WebDAV issue. Read more about it on Microsoft support sites.
The problem is not a lack of space in your Koofr account, but Windows incorrectly showing you how much space is left. The only solution for this problem is uploading your files to Koofr another way and not through the mapped network drive. Either you upload your files through the Koofr web app or through the Koofr desktop app with synchronization. Alternatively, you can mount Koofr as a network drive through rclone. This way you will not have this problem. You can find the instructions here.