XML Parsing Error: xml processing instruction not at start of external entity

[Scandal]

When we click Edit button of any post (so the Quick Edit form should displayed), I get this error on the browser's console:

XML Parsing Error: xml processing instruction not at start of external entity
Location: http://www.xxxxx.xx/ajax.php?do=quickedit&p=438
Row number 2, Column 1:

... the Quick Edit form is not appearing the the progress bar displayed permanently.

- I have try to disable hooks, but the problem still appears.
- I have this row on config.php: ini_set("display_errors", false); so I don't think it is a fatal error/ warning by PHP which brokes the xml normal syntax.
- I have informed that this appear starts after the move of the site to another server.
- We use vB3.8.4

Any idea to take a look?