Upload file JSON error (v2019)

I use PHPMaker v2019 and wamp server php 7.4.0 version
When I want to upload a file, there is an error:
SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON dataIn Chrome F’12 Network error: Syntax error: Unexpected token: ‘<’ ,"
<fo" … is not valid JSON

Could you help me, plrease?

What kind of file type did you upload? Are you sure you have allowed that file type to be able to upload to web server?

You may check HTTP response to find the actual server side error message.

I would like to upload pdf file. Yes, this file type is allowed. If change to PHP 7.3, it works. In PHP 7.4 it is an error.

arbei wrote:

You may > check HTTP response > to find the actual server side error message.

You may post the PHP error message for discussion.

After browsing and select the file I want to be uploaded, this message appears: SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data
When I click on submit, the chosen pdf file is not be delivered to PHP. The edited record is closed. When I click on this row in the list page, there is no uploaded file.I made a record file with Chrome development tool.
This is the part of the file where I want to upload the pdf. The error appears at this point{
“type”: “change”,
“value”: “C:\fakepath\1. számú függelék_INFORMATIKAI FELHASZNÁLÓI NYILATKOZAT.pdf”,
“selectors”: [
[
#x_d_egyseges
]
],
“target”: “main”
},