Firstly, there is no default password. It must be set on firstboot. I'm not 100% sure, but there may be a couple of possible causes of your issue.

The first possibility can occur if you are using a keyboard other than US-International. If that's the case, when you log into your server via SSH, it's possible that some characters (generally characters that are not used in US English) may be misinterpreted (thus stored as an alternate US-International keyboard mapped character). When you try to log in via your web browser, the keyboard characters will be correctly interpreted, thus they won't match the stored (misinterpreted) password.

The other possibility that occurs to me is that Nextcloud has an internal password checking utility. Our firstboot script should manage that and return the error message, which should note that the password is not accepted and ask you to try again. Perhaps there is some edge case where the password is accepted by the firstboot script, but not by Nextcloud? I strongly suspect that this is not the issue, but perhaps?

Assuming that you can log in via SSH, you could try again. Like this:

/usr/lib/inithooks/bin/nextcloud.py

That will interactively ask you for a password and should error if it's not accepted (as per the second possibility above). Alternatively, you can run it non-interactively like this:

/usr/lib/inithooks/bin/nextcloud.py --pass='ReallyGoodPassword1' --domain='www.example.com'

(Where ReallyGoodPassword1 is a really good password and www.example.com is the actual domain you intend to use)

I suggest trying to use at least 10 characters of just numbers, upper case, lower case and numbers. Note that if you use the non-interactive approach (the second example above), you should see the actual characters that will be used for the password, so that may be a better approach in this case?

Assuming that it succeeds, you can test the log in straight away (i.e. run the command in SSH, then test the login in your browser) so you can retry a different password if it initially fails again.

Good luck and please let me know how you go.