same issue, this happened to me some 30hours ago haha. i just fobbed it off and kept playing. I'm pretty sure my steps were:
talk to warden about downstairs
go downstairs, do the Grimoire side quest, leaving the sanitarium without talking to anyone else.
finish that side quest
door downstairs is locked
talk to Warden, who goes on about "what happened down there??" i was very confused and cant remember what option i chose. now i cant talk to warden anymore. my savegame is attached in that link. though this is from hours and hours after, i just left it and moved onto other things. https://onedrive.live.com/redir?resid=5DB2B0347EF9BD33%2116179