I just struggled with setting up external web hosting with MySQL for tile storage and i found out it was the sql user password that wasn't in a supported format for dynmap, the error reporting from MySQL_funcs.php indicated it did not support caching_sha2_password and had to use mysql_native_password which now produces a ton of error messages in MySQL server: "Plugin mysql_native_password reported: ''mysql_native_password' is deprecated and will be removed in a future release. Please use caching_sha2_password instead'"

I suggest a feature request for dynmap external web hosting to support this upcoming change in mysql as to avoid future headaches :)