Thanks for the suggestion, @stratacast. It actually solved the problem. I’ve had all kind of weird issues with gnome-keyring in Xfce on different distros (Arch-based ones, Debian, Tumbleweed and Xubuntu). I just ran
rm -v ~/.local/share/keyrings/*.keyring
and rebooted.
After that, I assigned a new password to the keyring, rebooted again and when asked if I wanted to unlock the keyring at session start, I said yes. And voilà. That was it. I can now launch the Nextcloud client without having to type my password over and over again.
Thanks for your help! I really appreciate it.