Strange Error

"421 Unable to read the indexed puredb file (or old format detected) - Try pure-pw mkdb"

Does anyone know what this means/how to fix it?

Server error, if not your server you must ask admin to fix it. If your server extracted from pureftpd FAQ:

After having made a manual change to /etc/pureftpd.passwd, you must rebuild
/etc/pureftpd.pdb with the following commands:

pure-pw mkdb

Thanks, problem solved.