Why does the Cisco webui doesn't accept my admin password, even when the serial console does?

I have a new Cisco SG350X (SG350X-48P-K9) switch. After powering it on I have logged in using the web UI with the default admin/cisco password. I have changed the admin password to something else. I’ve logged out and tried to log back in, but the password was not accepted. I was sure I had no typo, but so be it.

I’ve connected a serial/usb cable and started password recovery in the boot manager. Set the admin password, and also created a different user, with another password. After a reboot I’ve tried to login again to the web ui, but both admin and the other user’s password are not accepted. Tried both Chrome and Firefox.

Connected via serial again, and I can login as admin and the other user just fine! So the passwords are correct.

What is going on here, why is the admin password not accepted in the web ui?