The dropdown to choose the interval when configuring a "Multibranch Pipeline" only displays a single value "1d" . The HTML of the page only contains a single option in the select (checked with Web Developer Tools).
When reading the code (PeriodicFolderTrigger.java) it is apparent that many values such as 1m, 2m, 5m etc. should be allowed.
Whe modifying the value with the browser's Web Developer Tools I can for instance set it to "15m" and the value is correctly used, saved and displayed on the site. But then it is again the only value on display.
The latest change (July 2017) to anything related to this (as far as I can tell) was 853c294ae34fbd9f7156b51c56ee51a886702aee where the default value was set to "1d" (from being unset before).