Illegal string offset 'username' on MCP PM report pages

When clicking on either "Close report" or "Delete report" in „MCP/Reported messages/PM Report details“ and then confirming it by clicking on "Yes", an error message appears at the top of the screen saying:

[phpBB Debug] PHP Warning: in file [ROOT]/phpbb/notification/type/report_pm_closed.php on line 107: Illegal string offset 'username'
[phpBB Debug] PHP Warning: in file [ROOT]/phpbb/notification/type/report_pm_closed.php on line 108: Illegal string offset 'username'

followed by a few warnings - as seen in the attached "bug-report-pm-closed-1.jpg" screenshot.

This happens when phpBB is supposed to send an email to the reporter because he/she has „Your report on a private message is closed by a moderator“ notification type set to „EMAIL“ in his/her UCP.

Subsequently, the reporter receives the „Private message report closed“ email that does not display properly

and

, as seen in the attached „bug-report-pm-closed-2.jpg“.

Note 1: The email file used for this is called „report_pm_closed.txt“ which is new in phpBB 3.3.2. Interestingly, there has been a file called „pm_report_closed.txt“ for a long time that says more or less the same but has possibly never been used.

Note 2: The „report_pm_closed.txt“ file is used both when closing and deleting the PM report even though the email says „The report was afterwards closed.“ Interestingly, there has been a file called „pm_report_deleted.txt“ for a long time that says „the report you filed regarding the private message was deleted“ but has possibly never been used although it would be more fitting in case of deletion.