Certain date formats that are accepted in JIRA as Date/Month/Year format are not accepted by Portfolio's date picker. The problem here was that the year format was specified in upper-case Ys (e.g., dd.MM.YYYY) which is correctly interpreted in JIRA but causes the date picker in Portfolio not to open / accept manually input dates.

Workaround:
Change the upper-case Y to lower-case y

Original bug report:

As far as I know, this is already known bug, however, I wasn't able to find it. We are not able to choose date from date time picker in any section of JIRA Portfolio. Could you please update us on this bug, is this fixed in some version or we should still wait? Regards, Jelena