Hi there largeboss,
I have just checked the quest, and I was able to successfully complete "Crypt" (or well, Underground Chapel) and obtained the condition needed to complete "The Path of the Apostata, Part 5" and therefore, I was able to promote to Apostata with not issues whatsoever.
If the issue for you still persist please consider sending a ticket to the support system so they can gather the information from you such as character and other information so it can be properly looked into since just for what you said, it's extremely strange that the boss does not show up for you (but it does for me) and on top of everything you get stuck.
Also if possible, take a video (doesn't need to be a full video of the whole dungeon run, the start of the last stage until the issue happens would be enough) and either attach it to your ticket or upload it to YouTube and give the link on the ticket.
Love,
- Senakhtenre