Since when does this happen? Is this a fresh installation? Has this been an upgrade? Which MediaWiki version is this about?
Please see https://www.mediawiki.org/wiki/How_to_report_a_bug

I don't know since when does this happen. I suspect that the error is since installation of MediaWiki 1.24.1. This site exists a few years and was upgraded a few times.

Please enable error reporting and also set $wgShowExceptionDetails = true;

I've added to LocalSettings.php:

error_reporting( -1 );
ini_set( 'display_errors', 1 );
$wgShowExceptionDetails = true;

This does not alter error messages which I see in Apache2 log:

PHP Notice: Uncommitted DB writes (transaction from DatabaseBase::query (DatabaseBase::query)). in /var/www/withoutvowels/web/w/includes/db/Database.php on line 4266

Notices aren't exceptions and as such PHP doesn't provide stack traces (unless you install something like XDebug).

However, if you set up a debug log file in LocalSettings.php, notices will be logged there (including exceptions) as of MediaWiki 1.25alpha.

I tired it in my MediaWiki 1.24.1. There are no notices in debug file.

If it is really needed, I may create a new subdomain and setup an alpha version of MediaWiki there in order to make the trace.

Anybody having an idea what other info VictorPorton could provide here to track this down?
debug log file is empty.