Other than what was suggested above, this piece of information came up recently:

Here is a vid: https://youtu.be/DLACva2EEao?t=701

https://www.naeu.playblackdesert.com/en-US/Forum/ForumTopic/Detail?_topicNo=37038

Something to try, anyway.

Update as of 8/11-2023. 

I just finished this questline having experienced the first bug mentioned here. What I did to be able to reenter Syca's Glare, was to go back to the beginning with the teleport. Re-do the whole dungeon. So killing all the mobs and first 2 bosses required for all the different items. 
Once I had recreated the [ADV] Syca's Contorted Tentacle, it worked and I was able to go back into Syca's Glare to finish the quest. 

Despite the bugs, it was in my case very worth it as I got lucky with the quest reward chests. :) 

Goodluck

Help I'm having this exact issue, stuck on Syca's Glare and can't reset. Although I kill the boss the quest progression doesnt move forward. Is there anyway to complete this quest??

Can you show the exact quest youre currently on? And what happens when you abandon and re-take the quest?

Hello, thank you for your assistance, I have been experiencing some serious OCD trying to finish this, haven't slept much.

The name of the quest is [Atoraxxion] Syca's Glare - Completion Target: Syca's Glare Sol Magia.

My first time doing the quest I don't remember for sure but I believe I got lost somewhere between the squid/octopus and the whale boss. So I restarted the quest and re-entered the dungeon by changing servers. After an hour of searching I found my way back to Syca's Glare, and was able to kill the whale boss. After the whale boss died, I interacted with the Sol Magia NPC and when I tried to click the button which said [Solo] - (go to final boss room), the NPC responded and said 'error connecting' or something along those lines. However the portal did open up beside the Sol Magia and I walked in and killed the boss. Still the questlog did not move from what I said originally.

I spent another hour redoing the quest twice from the beginning and still the questlog is stuck at Syca's Glare - Completion Target: Syca's Glare Sol Magia.

I'm willing to try anything to get this quest done and finished. Thank you again.

Having the same problem. No way to reset the dungeon so I can get the key parts. Terrible design. 

Just experienced this problem and solved it. What I did was talk to the talk to the Altar that spawns the second boss and select to spawn it AGAIN. The game said that I already had killed it and can move to the next area. After this, the Altar dissappeared and the lightning wall blocking the path also dissapeared. Hope this makes sense and helps you!