SPOILERS || Quest support

[Thaliel]

finally advanced to the encrypted message, but I absolutely do not understand how I am supposed to use CAROL as a basis for decoding, this is way too confusing

CAROL is 'the key' and just put in front of the normal alphabet in two different ways:

First way it is : go on with the letter that would follow the key's (CAROL) last letter (L) in the alphabet (=M)
Second way is: go on with the normal alphabet (=would be A, but since you have to delete all letters that the key has (C+A+R+O+L) to avoid double letters, you skip A and go on with B)

if that doesn't help, read on^^
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Now you write down the whole new alphabet:
(I shorten it by ...)

First way: CAROLMNO...XYZ
Second way: CAROLBDE...XYZ

New alphabet stands for the code-letters. Write the normal alphabet under it in a way you see which code-letter stands for which normal letter.
Now take the quest code and happy transforming^^

sorry, I still don't get it
I have
QE QD XFB OFEJ CWO YBQGQULNL EX ABLCEPL UQML QWEX C WLH CNL XM LWUQNPELWVLWE

no idea how to find out what stands for what