I suggest you to follow Microsoft recommendations on how to restore the operating system.
NOTE: These links are being offered for your convenience and should not be viewed as an endorsement by Intel of the content, products, or services offered there.
Thank you, @mikec_intel, but unfortunately neither of these options works.
"Last known good configuration" has a backlog of limited size (so I don't have a "good" configuration available any more), and it's not working all together for full system image swaps like the Windows 1511 update was.
The second link isn't exactly helping either, neither does the 1511 update show up in the list of updates (since it wasn't just a regular update, but entirely new system image), nor would it be wise to uninstall the RST driver when the system is configured to boot from a disk attached to that controller. Well, and I already tried the latter one as well. Removed the driver, reinstalled it from a rescue system with DISM, and it was still broken.
I'm trying a rollback to an earlier system image now (via Settings -> Restore -> "Return to ealier build"), which is only possible as long as the previous Image still exists as "Windows.old". Let's see if that fixes things.
But it's still not solving the initial problem: The migration procedure conflicts with the RST driver.
OK, do not attempt to roll back to a previous system image, it's breaking even more things. No even the RST installer is working any more after a rollback.
Something is really broken about this whole migration/rollback process introduced with Windows 10. And I'm not yet sure what it is, respectively if MS or Intel is responsible for the issues.
It is possible to uninstall and delete the current RST driver from your computer from programs if you created the RAID from control-I menu. Then, install the latest driver provided with your motherboard manufacturer.
If you created the RAID volume with our software; verify if you already have the latest BIOS version, latest chipset and Management Engine driver provided with your motherboard manufacturer.
I had already tried all of these options as well. The RAID was generated with the software (hybrid cache), not from the ctrl+I menu, and I did already try dismantling and reassembling the RAID multiple times, including reseting all involved disks to non-RAID, and the reassembling. Yes, that "helps" in such way that I can access the disks individually, but the cache functionality is still non-functional.
The motherboard manufacturer isn't providing a Windows 10 compatible version of RST, and the "most recent" BIOS version is from 2014.01.21 (GIGABYTE H77M-D3H with H77 chipset). Chipset and Managment Engine drivers provided by Gigabyte were from July / September 2015 respectively.
Well, I went with a clean reinstall now, based on the 1511 image. The driver was just broken beyond repair by the upgrade. Tested it, after a clean reinstall, RAID would have worked again. But I will definitely never use an Intel chipset for any type of RAID under Windows 10 again, that software is way to unstable.
And unless either Intel or Microsoft finds and addresses the root cause in the upgrade procedure, this will happen again with the 1605 system image in May 2016.