If I may add another data point here:
I was seeing this on a laptop running Arch Linux when I logged into an LXQt session, but not when I logged into KDE/Plasma.
Under LXQt, I found if I closed the Nextcloud app without logging in again, unlocked KWallet manually, then re-launched Nextcloud, it found the existing login credentials from before.
I finally solved it by enabling the KWallet PAM Socket Connector in my LXQt Autostart settings. So it opens the wallet as I log in, and then Nextcloud finds the login tokens just fine.
So it seems to be an issue with a keyring manager not running until after it’s needed.
That said, I also have the same symptoms on a Macbook. I’ll have to check on the keyring settings there next time I open it up and see if it’s the same underlying issue.