MyBB Community Forums - Can't use Quick Edit button - the forum returns an empty XML form

MyBB 1.8.26, Nginx 1.16.1, PHP 7.3.27. I tried to change PHP version and switch to Apache, but nothing has changed.

If I do quick edit a post - the forum returns an empty XML form and says only "Loading...":

<?xml version="1.0" encoding=""?><form>

</form>

A web browser says that an error is in xmlhttp.php:1:31 and showthread.php:1:31:

XML Parsing Error: XML declaration not well-formed

Quick Edit button's response headers:

HTTP/2 200 OK
server: nginx/1.16.1
date: Thu, 25 Mar 2021 17:32:31 GMT
content-type: text/xml; charset=
x-powered-by: PHP/7.3.27
expires: Sat, 1 Jan 2000 01:00:00 GMT
last-modified: Thu, 25 Mar 2021 17:32:31 GMT
cache-control: no-cache, must-revalidate
pragma: no-cache
strict-transport-security: max-age=31536000;
content-encoding: gzip
X-Firefox-Spdy: h2

The URL is:

https://myforum/xmlhttp.php?action=edit_post&do=get_post&pid=1219&id=pid_1219

There are no errors in the web server's log.

How to fix it?

you need check browser console error log.

(2021-03-26, 09:17 AM)martec Wrote: [ -> ]you need check browser console error log.

The problem is not in the web browser.

(2021-03-27, 09:13 AM)r00tk1t Wrote: [ -> ]
(2021-03-26, 09:17 AM)martec Wrote: [ -> ]you need check browser console error log.
The problem is not in the web browser.

Umm - he didn't say that. Developer tools...

(2021-03-27, 11:05 AM)nixer55 Wrote: [ -> ]Umm - he didn't say that. Developer tools...

yes...
anyway... if no error on Developer tools or browser console... try diable all plugin and made test again....
there is a high chance that a php script or plugin is blocking the execution of the code.

Thanks everyone. I just disabled and enabled each of the plugins, and now the quick edit button is working. I don't know what plugin caused the error.