This has been annoying me on Linux Mint 22 Cinnamon too.
I installed the KDE Wallet with:
sudo apt install kwalletmanager
Signed into the popup that kept appearing, then opened KWalletManager, go to the Applications tab, highlight Nextcloud, click Change Password and then just click OK without entering a password.
I’ve restarted a few times and the popup doesn’t appear anymore, don’t know if not having a password is a security risk though.
If I entered a password in KWalletManager then it would still popup every login, only not entering a password has seemed to stop it from popping up.