Once you go back to the grove after accepting to help her, you should get the cutscene Ragnarok mentioned where Zevlor hurries you inside at the gate itself. If you don't get that cutscene, you want to reload and follow Minthara to the entrance before going back to the Grove. When i fast traveled immediately after accepting her quest in her room, Zevlor's scene never triggered no matter what I did afterward.
Sometimes doing this isn't enough. I followed her and still got no cutscene with Zevlor.
Reload, followed her again, nothing.
In general, this is a quest Larian really needs to take some time to polish because it can really screw up your entire progress.
