The quest Celebrating Solstice doesn't progress past step 3 on my second character (first character has fully kindled the armor set and completed the Flamekeeper title/seal).
I have done as the quest asks (first run with only class item equipped, second run with a full set) and it won't complete upon looting the ash from the bonfire at the end of the activity.
I believe this to be a bug.
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Maybe it’s because you’re holding 500 silver ash (max you can hold). Try to spend it in order to gain more on your second or third character