Theming apps issue: thumbnail icon not shown : /apps/theming/icon?v=200 return 0B

Hello, I have an issue when I share a public link on whatsapp the thumbnail icon not showing here the description of the issue:

Steps to reproduce

1.enable theming apps
2. change a color and upload new log
3. share a public link in whatsapp or any social media

Expected behaviour

the thumbnail icon should be appeared when share a link on whatsapp

Actual behaviour

when sharing a link, there is no preview of thumbnail icon

Operating system:
FreeBSD (FreeNAS)
Web server:
nginx version: nginx/1.16.1
mysql Ver 14.14 Distrib 5.7.29
PHP version:
PHP 7.3.15
Nextcloud version: (see Nextcloud admin page)
nextcloud 18.0.3
Updated from an older Nextcloud/ownCloud or fresh install:

Dear Community,

can awesome expert help me? I don’t how I solve it.

thanks in advance!