Incorrect activation link generated

Hi, latest version of WordPress, MU install, no extra themes or polugins.

Subdomain install.

Regsiter and I get sent an email with the activation link but it is incorrectly formed eg:

https://test2.domain.com/wp-activate.php?key=5e069d2f0ca2d39d

This gives the
“An error occurred during the activation
Invalid activation key.” error.

However, if I take out the subdomain eg:

https://domain.com/wp-activate.php?key=5e069d2f0ca2d39d

It works fine.

Bug? Is there a workaround please?

Many thnnks

Clive