Windows won't boot anymore

lastly I had the problem of windows rejecting to boot anymore. Instead of windows a nice UEFI repair tool greeted me stating, that Windows won't boot anymore.

Problem description

For those who will run into the same problem and trying to fix it, I want to make a small guide to show that you are not alone.
Since the UEFI diagnosis tool performs a diagnosis of system files and creates a log, you can see, what is missing.
The file is located in C:\windows\System32\LogFiles\Srt\SrtTrail.txt

At the end of the file there was written: "C:\efi\microsoft\boot\resources\custom\bootres.dll is corrupted"

The problem with BitLocker

If you wonder how BitLocker encrypts your drives, there is a simple answer: It encrypts everything making these drives unable to mount in any OS. This makes it even worse to fix any errors at startup.

The solution

for BitLocker

The tool manage-bde which is a core program of every windows (and windows recovery) will help. The command manage-bde -unlock <drive letter> can be used to unlock a drive. There are some parameters how you can unlock the drive. Either a password or a key located on a USB drive.
If you ever wondered how the key is stored: It is a file with the extension .BEK

For the bootloader

If that happened, it is relatively easy to fix. I have to admit, that I did not find the solution myself, but it took me several hours.

1. Start the computer in recovery mode with the command prompt.
2. Open another cmd with the command start
3. Type in the command: diskpart. This will start window's disk utility.
4. List all volumes by typing list vol. There should be some volumes. But There should also be a volume without any letter and a size of approx. 500mb. This is an EFI partition. We will assign a letter to that.
5. Select the volume by typing select vol <number of the volume>
6. Now, we will assign a letter to that volume by typing assign. Now the volume will have a letter and we can see it with list vol.
7. The next two steps are for validation. Skip those if you know what you are doing
8. CD into the new partition with typing <letter of partition like shown in diskpart>: in the other command prompt that you opened at startup.
   7 . If you have selected the right partition you should see a folder called EFI
9. Now, we will rebuild the efi partition (which is the corrupted part of the OS)
10. In diskpart select the volume containing the EFI partition.
11. Format it using format <letter of partition>: FS=FAT32 QUICK OVERRIDE
12. exit diskpart by typing exit
13. Rebuild the bootsector by typing bcdboot <volume label of windows installation>:\windows /s <volume label of efi partition>: /f UEFI

Now the bootsector should be rebuilt and you should be able to reboot and boot into a fresh windows