Have you looked into Edit > Preferences > General for InDesign CC 2015 and have you experimented with the first two checkboxes that are now there?
Hope this helps,
Thanks for the input.
I unchecked the "Show "Start" Workspace When No Documents Are Open but that didn't work.
I open a book file, put the panel on the right (along with layers, links etc) and when I open a document from the book, the panel disappears...
besides that, all the panels minimize when I open a new document. (by the way, the "Auto-collapse Icon Panels" is off)
Then I unchecked the box "Auto-show hidden panels" at Edit > Preferences > Interface and seemed to work, but enabling the "Start" Workspace again closes the book panel again.
It seems to be some kind of bug.
I was wondering if this happens to anyone else too, and hope for an update.
If not, I probably should uninstall and install again to see if that works.
Trashing the Preferences worked for me as advised in another question on this forum. Hold down Ctrl-Shift-Alt immediately after Opening the software. You have to be quick.