Answer from castaglia (main proFTPD developer) : Now, starting with MySQL 5.7, the "Backend" SQLAuthType will no longer work. The alternative is to start using the MD5/SHA1/SHA256 (or other) SQLAuthTypes provided by mod_sql_passwd.
Complete answer here : https://forums.proftpd.org/smf/index.php?topic=12043.15
Basically, Backend SQLAuthType cannot work with MySQL >= 5.7.
Answer from castaglia (main proFTPD developer) :
Now, starting with MySQL 5.7, the "Backend" SQLAuthType will no longer work. The alternative is to start using the MD5/SHA1/SHA256 (or other) SQLAuthTypes provided by mod_sql_passwd.
Complete answer here : /forums. proftpd. org/smf/ index.php? topic=12043. 15
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Basically, Backend SQLAuthType cannot work with MySQL >= 5.7.