Date Field format error in Selenium Test


since the upgrade from 7.0 to 7.1 my selenium tests fail when inserting dates.
There seems to be some changes in the way the DateField validates. When I input the date value manually, it is ok and the valid date is accepted.

When my selenium test inserts it (sendKeys method of selenium), it tell me that the format is not valid.
This seems to be a frontend problem, debugging it shows the following behaviour:
In the method DateField.changeVariables the calendarFieldChanges variable stays empty. This is a result of the variables map having all relevant resolutions (in my case day, month, year) set to -1.
Why these fields are set to -1, I have no idea. Perhaps someone with more knowledge could help or point me to the right place.