[TYPO3-user-registration] sr_feuser_registration 3.1.3 additional field display format

[rsbrux]

I have been trying to upgrade my typo3 4.5 installation from sr_feuser_registration 2.6.1 to 3.1.3 for months now.
One of the minor inconveniences is that my multiple select fields (e.g. hobbies, languages) are not displayed correctly.
Under 2.6.1 I had a collapsed field showing "<m> of <n> selected", which would pop up the selection list when clicked.
According to the Firefox debugger, the corresponding styles come from the class ui-multiselect in jquery.multiselect.css
In 3.1.3, the corresponding field shows 5 lines of the list, and has a generic style from main.css.
How can I find out and fix what is wrong?
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Typo3 4.5.41
PHP 5.3
mySQL 5.4.39
Windows 8 x64