I executed the following command and I was able to reindex my records.
sudo koha-rebuild-zebra -f -v instancename
I just found that the problem is related only to Greek characters. The
English titles can be sorted in alphabetical order as it should. But when
the titles are in Greek the sorting doesn't seem to work. Maybe a bug?
Apparently, 'gr' (for 'Greek') is still missing from the options to the
relevant configuration variable (ZEBRA_LANGUAGE) in
This will particularly affect newcomers since they will be unaware that
such a language option for Zebra exists.