Steps to replicate the issue:

• Create account [[user:Test+]] (any username with + character – plus sign).
• Log-in as Test+
• Post a message mentioning another account you own.
• Log in as your mentioned account.

What happens?:

• The notification link contains “+” (plus) instead of “_” (underscore). (toast notification: “This comment could not be found on this page, but it does exist on the following page:”. The provided “following page” is still a broken link with same issue.)
• The reply button for the message is also broken (error dialog: “Could not find the comment you're replying to on the page. It might have been deleted or moved to another page. Please reload the page and try again.”).

See real world issue.

What should have happened instead?:
The notification should be properly linked to the comment. The toast notification warning should not display.
The reply button should work properly.