The new phpbb/file_downloader.php file does not define a user agent when the guzzle client is created. That means the default guzzle user agent is sent, which is blocked by several shared hosting companies due to guzzle oftentimes being used by PHP-based scrapers. It is likely that some of the issues currently being reported on phpbb.com are due to this bug.

Adding the following to the aray that creates the client will resolve the bug.

'headers' => ['User-Agent' => 'Mozilla/5.0 (compatible; phpBB ' . PHPBB_VERSION . ')']

Set as a blocker because this should ideally be fixed (much) sooner rather than later.