Started to figure out the temp file part.
.part extensie is added by Firefox, in case of: a partly download or the right sources are not installed on the computer or a virus or the file is damaged. In this case the right app is on my computer: firefox. So naming and add the .part extensie (gaSCA65C.odt.part) does Firefox.
Cleared the local temp folder. The issue still exist after clearing temp folder.
I prepared the server for error logs using the instructions mentioned https://perishablepress.com/advanced-php-error-handling-via-htaccess/#comments].
Later today, I hope to have a PHP error log …
I will keep you posted, thank you very much for helping!