Many thanks to Vitaly from the Russian-speaking segment of the forum
(see here)
The defect was self-resolved after making the following changes to moodle/lib/moodlelib.php:
/*
if ($localewincharset) {
$datestring = core_text::convert($datestring, $localewincharset, 'utf-8');
}
*/
/*
if ($localewincharset) {
$datestring = core_text::convert($datestring, $localewincharset, 'utf-8');
}
*/
P.S. I previously posted a fix request for MDL-76598, but I hope that the Moodle developers will fix this defect as soon as possible.