Fusion Drive lost space after bootcamp-fail. resizeStack throws an error. What can I do?
Since I wasn't able to resize the stack and seemingly no one was able to help me, I decided to to this:


Make an image of the Fusion Drive Volume and save it onto an external hard drive
Delete the Fusion Drive
Rebuild the Fusion Drive
Install OS X to let it create the Recovery partition
Boot OS X, run Bootcamp Assistant
Run the Windows Installer WITH NO EXTERNAL DRIVES ATTACHED (!)
Restore the image to the Fusion Drive
Be happy after 10 hours of work


Not the best procedure to do but as this was the only thing that seemed to help, I did it.

For recreating the Fusion Drive I used this answer from klanomath:
https://apple.stackexchange.com/a/168744/109455

I have a working setup with OS X El Capitan and Windows 10 now.