Error on download zip backup

[lotrdocker]

Hi,

In the backup section, when I click on Download Zip Backup, this message appear :

"There appears to be a problem with this site.

It's possible that http://xxx.xxx.xxx.xxx:xxxxx/index.php/backup/user_export is experiencing a temporary issue or has been moved.

Error code: 500 Internal Server Error

The site may be temporarily unavailable or overloaded. Please try again later."

The version is 2.8.6, it's a local installation and I deliberately removed the beginning of the URL.

73's.

[magicbug]

What's the PHP error? I'd suggest in index.php turning dev mode on