'Error 2' response when handling form-multipart file upload

[Dramacydal]

When sending form with one and only file input, mpcmf responds with text/plain 'Error 2'
If any other text input is added - error does not occur.

Looks like error comes from
vendor/react/http/src/Server.php:290:

which is called from
vendor\react\http\src\Server.php:187

Exception body here has message: array_merge(): Argument #2 is not an array

Request example:
Request headers:

Host: ******************
Connection: keep-alive
Content-Length: 16774
Pragma: no-cache
Cache-Control: no-cache
Accept: application/json, text/javascript, */*; q=0.01
DNT: 1
X-Requested-With: XMLHttpRequest
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/83.0.4103.61 Safari/537.36
Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryTiLFo6nR9Bo7ppsc
Origin: https://*************
Sec-Fetch-Site: same-origin
Sec-Fetch-Mode: cors
Sec-Fetch-Dest: empty
Referer: https://*************
Accept-Encoding: gzip, deflate, br
Accept-Language: en,en-US;q=0.9,ru-RU;q=0.8,ru;q=0.7
Cookie: *****************

Form data:

------WebKitFormBoundaryTiLFo6nR9Bo7ppsc
Content-Disposition: form-data; name="upload_file"; filename="Импорт клиентов_18.05.2020.xlsx"
Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet


------WebKitFormBoundaryTiLFo6nR9Bo7ppsc--

Response:

Error 2