Account creation with email fails on Firefox (works on other Discourse instances)

Connecting from a remote location so I don’t have the bandwidth to install additional browsers to test.

Using Firefox 90 on Ubuntu 21.04, account creation on help.nextcloud.com using an email address fails with error message:

We cannot detect if your account was created, please ensure you have cookies enabled.

Cookies are indeed enabled, and I have tried creating accounts with email on a discourse server I run myself, as well as on try.discourse.org. Both work OK.

I was able to use Discourse to sign in, but there may be a problem with the server config.