@Christian-Ehrlicher said in QPSQL: authentication method 10 not supported:

It was meant
a) as a test if it helps

True, that should have given more information - but now the fog is even denser. The result, with the same test script: same error message with md5 in all the "host" or "local" records in pg_hba.conf, after restart of PostgreSQL. It works as expected with "trust" in the same places.

I should perhaps add that I don't use QtSql with PostgreSQL databases in earnest. I just have some test scripts - and it's quite possible that I didn't run them for every new PostgreSQL or PyQt version.

I've had problems with QtSql using PostgreSQL or Interbase/Firebird before, but this is the first time I couldn't even find a bug report for them.

Well, I have discovered my problem.
I was trying to read the file before closing it:

QString cadenaimportar = "SELECT importar_copy('" + tabla + "','/path/conceptos.csv',)"; ...... file.close();

And it would be:

file.close(); QString cadenaimportar = "SELECT importar_copy('" + tabla + "','/path/conceptos.csv',)"; QSqlQuery consulta(db); if(consulta.prepare(cadenaimportar)) { consulta.exec(); } qDebug()<<consulta.lastError()<<"--"<<consulta.lastQuery();

I have usually seen processEvent called from the main thread directly which forces processing of events however calling it from a different thread is the first time. As you can see there's some strange interaction. I currently wonder whether you have an event that is processed by the wrong event loop.

Thank you very much @SGaist and @raven-worx for helping me out. I fixed this issue by using runtime libraries from this project:
https://sourceforge.net/projects/postgresql-mingw-w64/