Properly check if TMP file exists - PHP 8

after switching php of my phpbb 3.3.3-board from 7.4 to 8.0.3 I cant upload files anymore. I get the error message "Error parsing server response."

All folder have the necessary rights, also when switching my sever back to php74 everything is fine. Any idea what this could be. Anyone else can confirm / reproduce this?

All extensions have been switched off, file-type doesnt matter.

See also discussion here: https://www.phpbb.com/community/viewtopic.php?f=661&t=2589106