Problems with IE11 - Window doesn't stay open.

Hello @community!

We are using Pickadate code in version 3.4.0 and it works like a charm for FF and Chrome. Unfortunately many of our users are accessing our site in the office and there they are using Internet Explorer. We encountered a problem with Internet Explorer that one may not insert a date as the Pickadate closes the same moment after opening and does not stay open as it would supposed to.

If you would like to have a look at the problem, please feel free to access our test site using IE11 (or possibly any other version, we couldn't verify this) :
http://www.test.lehrmittelperlen.net/jetzt-anmelden

Please register any values at the first page and go to the second page. There you should try to insert your birth date. As you will see... it is impossible :(

We would really appreciate any help to get this up and running!