The greek calendar translated javascript files are corrupted and result in javascript runtime errors which prevent the opening of the pop-up window. The problem probably lies in the
tlassian-jira\includes\js\calendar\lang\orig\calendar-el.js file which contains an extra space at first line before the comment // ** I18N. So the encoded calendar file in the parent folder gets some extra invalid chars \ufeff// ** I18N. When we've removed the extra chars the pop-up worked as expected.