Steps to reproduce:
You need three user accounts: User1, User2, and User3. (You don't actually need access to User3's account, but the account needs to exist.)
Log in as User2.
View your profile, go to Manage Pseuds, and add a new pseud with the name User3.
Log in as User1.
Create a work with a brand new series and with "User3" listed as a co-creator.
Because "User3" is ambiguous, you will be prompted to clarify whether you meant "User3 (User3)" or "User3 (User2)." Choose one – it doesn't matter which – and finish posting.
The posted work does not belong to any series.
What should happen:
The posted work should belong to a series.
(Note that this is not identical to AO3-5606, which deals with adding ambiguous co-creators directly to the series.)
Added a new work in a new series on testy, with co-author Nary. It asked me to clarify, I picked one and posted. Work correctly appears in a series! Looks good.
Went to post a new work with the co-creator “testy” and the new series “5662 Series.” When I pressed “Post Without Preview,” I was taken back to the form with an error saying in part, “Invalid creator: The pseud testy is ambiguous.”
I selected a user who had the pseud “testy” from the dropdown (the user I chose was “orphan_account”).
As I scrolled down, I looked at the series portion of the form and noted “5662 Series” was selected from the “Choose one of your existing series” dropdown.
I pressed “Post Without Preview” and… was taken to a blank white page, with the URL https://test.archiveofourown.org/works in my browser’s address bar (Safari 12.1.2). When I checked my work listing, my new work was definitely not posted.
The series was created, though: if I go to the post new work page, it’s in the dropdown that lists my series. The URL is https://test.archiveofourown.org/series/63623 (accessible only to logged in users).
Something’s not quite right.
ETA: Because I left the white page with https://test.archiveofourown.org/works open in a tab, just for fun, I pressed refresh in my browser. It asked if I wanted to send the form again; I said yes. Nothing happened. I did it again, including saying yes. The work was created that time and it was added to the series, which is no longer empty and no longer restricted to logged-in users. It was all rather odd.
The white screen was unrelated to this particular activity, and trying again worked just fine. Ready to go!