I changed from RPI Stretch to Buster and now get the following error when I try to initially connect to the database:

(a) when I connect from Buster to a DB on a Stretch OS:
mysql error '2013' - Lost connection to MySQL server at 'reading authorization packet', system error: 11

(b) when I connect to the local database on a Buster OS:
mysql error '2013' - Lost connection to MySQL server at 'reading authorization packet', system error: 104

This is from code which looks like:
::mysql_options(state, MYSQL_READ_DEFAULT_GROUP, app_name.c_str());
connection = ::mysql_real_connect(state, host.c_str(), user.c_str(), pass.c_str(), db_name.c_str(), 0, 0, 0);
log_last_error();

It worked as long as I compiled and run it on Rasbian, Stretch but since I moved to Buster, I get this error. I found some pointers on the web which claim it had to do with too short a timeout, but increasing connect_timeout did not help.