Nils, please try again after restarting your PC/laptop. If this doesn't resolve your issue, can you disable your anti virus/malware protection temporarily?

If the problem still persists, you could export the website to disk and then try filezilla or similar to see if this resolves the problem?

Do you have more information about the error,
Domain name, PrintScreen of the error in the ftp report of wx5
Is there enough free space on the server,
the error occurs repeatedly with the same or different files during multiple uploads.
If the error persists after the automatic reload of the file that triggered the error.

JJ.