Singular

When running apt-get update on Debian 9 today I get:

The server refused the connection and said: OOPS: vsftpd: cannot locate user specified in 'ftp_username':ftp
Ign:33 jim.mathematik.uni-kl.de/repo/debian9 stretch/main amd64 Packages
Fetched 109 MB in 28s (3,878 kB/s)
Reading package lists... Done
W: The repository 'jim.mathematik.uni-kl.de/repo/debian9 stretch Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
in 'ftp_username':ftp
E: Some index files failed to download. They have been ignored, or old ones used instead.
Reading package lists... Done

I removed some ftp colon slash slash from output above since I cannot post URL here.

Also, Debian 10 is released on Saturday 6 July 2019. Please consider adding a repo for it as well.

Thanks for the report, it is fixed now.