Exactly in the same situation as well, the door to Solemn Path is not glowing blue and I can not use it. If I go from the south via the burning forest I can access Heorot’s and see his death but this does not trigger the end of the quest so back to square one.
Somehow, I found a way to “fix” it. Seems like the previous steps of the quest weren’t fullified…
My partner made a second character and did the story again, he then invited me to when he reached the Gates of Solarum and we did the main quest again together.
I noticed several “?” marks on PNJ, like I didn’t get the quests.
We retrieved the Lance and spoke to Grael, and when back to Heoborea to talk with Yulia. The Solemn Path was finally open.
I suggest you check in-game if the previous steps (Entering Tomb of Morditas, Defeat Yrun, Taking the lance, Speaking with Grael, etc…) were fullified.
However, I don’t know if you can do this in a solo game… Also, I don’t know how this bug occured.