All uploaded files are 0 bytes

[NMike71]

Hello and thank you for this great project, but unfortunately I have the following problem:

The uploaded files are created in the target directory, but always have a size of 0 bytes.
In the 'upload_tmp_dir' i can see that the file chunks are being uploaded.
There is no error message; all uploads are reported as successful. {"status":"success","info":"file upload successful"}

TFM otherwise works perfectly. Creating, modifying, and deleting files is error-free.
PHP upload limits are adjusted. Even very large files (xGB) are uploaded.

TFM is running on a NAS with PHP 5.6.40 (unfortunately, no newer PHP version is available).
Does anyone have any idea what could be causing this?
Thank you,
Michael