@webmin Try to add --no-ssl-cert-verification right after pgloader as shown below.

pgloader --no-ssl-cert-verification mysql://DBUSER:DBPASS@localhost/DBNAME postgresql://doadmin:PASSWORD@"stuff.db.ondigitalocean.com:25060/DBNAME?sslmode=require"

Btw I had to wrap the server address of the PostgreSQL server into quotes on macOS. Otherwise I was not able to add the needed sslmode option at the end without triggering an error.

Hey there @webmin,

Have you tried running the following instead:

pgloader mysql://DBUSER:DBPASS@localhost/DBNAME postgresql://doadmin:PASSWORD@stuff.db.ondigitalocean.com:25060/DBNAME

Note the removed ?sslmode=require from the Postgres connection.
The error implies it is trying to force an SSL connection but failing because the SSL certificate that is present is only a self-signed one and not a properly validated one.

Let me know how it goes!
- Matt.