Here's the activation email body:

>>> email >>>
Welcome to ...

Please keep this email for your records. Your account information is as
follows:

----------------------------
Username: xxxxxxx

Board URL: https://www.example.com:80/phpbb
----------------------------

Please visit the following link in order to activate your account:

https://www.example.com:80/phpbb/ucp.ph ... =JOV50ED51

Your password has been securely stored in our database and cannot be
retrieved. In the event that it is forgotten, you will be able to reset it
using the email address associated with your account.

Thank you for registering.

– Thanks, The Management
<<< email <<<

as you can see https:// is specified along with port:80, which leads to a problem.

I've commented out line # 2207 within file phpbb/includes/functions.php to temporarily avoid the problem.