PHP Warning: Missing boundary in multipart/form-data POST data in Unknown on line 0

I'm logging every error on my server into a file and time to time I'm solving the cause of them.

I have several log entry with the PHP warning:

[11-Jun-2017 10:49:27 Europe/Berlin] PHP Warning:  Missing boundary in multipart/form-data POST data in Unknown on line 0
[12-Jun-2017 08:58:27 Europe/Berlin] PHP Warning:  Missing boundary in multipart/form-data POST data in Unknown on line 0
[13-Jun-2017 05:57:19 Europe/Berlin] PHP Warning:  Missing boundary in multipart/form-data POST data in Unknown on line 0
[13-Jun-2017 05:58:01 Europe/Berlin] PHP Warning:  Missing boundary in multipart/form-data POST data in Unknown on line 0
[14-Jun-2017 05:42:27 Europe/Berlin] PHP Warning:  Missing boundary in multipart/form-data POST data in Unknown on line 0

The problem that I have no clue what is the trigger for this. So my question is that how and where should adjust my log to store the request headers with this warning? Any other idea what could help to see what produce this warning?

Server is running: Ubuntu 16.04.2 Apache/2.4.18 PHP 7.0.18

Typically, the trigger for this kind of thing is a file upload or a form with a large amount of data.

If all you want is the next steps in how and where to adjust your log, I would start by logging information in PHP for the superglobal $_FILES (and maybe even $_POST, although tracking POST data over a period of time will result in a large amount of noise).

This question on StackOverflow provides some more of the programming background and possible solutions once you narrow down the culprit further.