I apologize if there's already an answer to my problem but if so, I honestly can't find it.
I seem to have messed up my configurations somewhere since I can't set date/time for my sprints:

I've cross referenced the date format in Look and feel and under System > Advanced settings (https://confluence.atlassian.com/jira/changing-the-due-date-input-format-192536.html#ChangingtheDueDateInputFormat-datepickerformatsConfiguringdatepickerformats):
Look and feel:

Advanced settings:

As far as I can tell both settings are the same.
On the other hand, the waring says that the "picker" isn't supported by the new issue view. And think it's here somewhere (new issue view) I might have changed something but can't find where!
I suspect I changed date/time format in issue view to "Swedish" format prior to changing preferred language to Swedish.
Now my personal and global settings are set to Swedish.
Any tips on how to find where I can restore my suspected date format changes in the new issue view?