When a user receives an email notification and clicks the link, they are taken to the boards login screen. After logging in, the receive the message,
"The requested topic does not exist."
After logging in, if you click the link, it works just fine.

When I posted this in support forum at phpbb.com, that boards behavior was the same. Someone there said the following for the reason

"It is due to the way the redirect url is being parsed. the & is being converted to &amp3B which is the HEX equivalent of ; "

Not sure if accurate or not.

It also happens at phpbb.com support forum if you view a thread, click post reply, if you are not logged in you are taken to login screen.

After logging in, you are not redirected correctly and receive the error

" the requested topic does not exist"