I tried to move the file /usr/local/share/lua/5.2/DBI.lua away, then
the error message changes to:
Dec 05 11:57:35 sql error Error in SQL transaction:
/usr/share/lua/5.2/DBI.lua:53: Cannot load driver MySQL. Available
drivers are: PostgreSQL
So now it tries to load the "official" source. But the mysql driver is
still not found. What still confuses me, that the Postgresql driver
installed from the Debian repository is recognized correctly. So it
seems to look at the right places but something seems to be different
if it comes to the mysql driver.