Stuck Pre-load Adjuster

[seppvonbayern]

The pre-load adjuster on my F650GS seems to be stuck (i.e. wheel is only turning in place but pre-load is not changing) after my son cranked it up to the max for a weekend ride out. Any idea what could cause that and how this can be fixed?
Thanks for any help you guys have - SeppvonBayern