I fixed this last week, forgot there was a ticket for it.
The User-Agent HTTP header is presently "X-HDS-Client", but should be "HaikuDepot/x.y.z" where "x.y.z" is the version of the HaikuDepot application. Stephan; are you able to see if this is OK for me?
When BSplitLayout::SetItemCollapsed() is called, the expected argument is a boolean indicating whether the item should be collapsed or not. However, internally this value is mapped verbatim onto BLayoutItem's "visible" setting, which has exactly the opposite meaning, as a result of which SetItemCollapsed(x, true) has the opposite of the desired effect.
Not enough information, at the very least the syslog is needed. Possibly a dupe of #12034. Please reopen if you think it's not.
Fixed in hrev49477.