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Setting $wgDebugDBTransactions = true; gives the error:

Catchable fatal error: Object of class mysqli could not be converted to string in \includes\db\Database.php on line 4110

Printing the callers before the (implicit) toString gives a sprintf in SqlBagOStuff as caller:
wfDebug( sprintf( "Connection %s will be used for SqlBagOStuff\n", $db ) );