I am facing issue with a odoo invoice formatting. It's not happening every time but sometime while sending bulk invoices to customers over email Odoo invoice format breaks and a pdf with no formatting is send to customers. 

There are 2 different kind of logs I can see when there is problem and when there is no problem in the format:
1. Invoice format works fine: odoo.addons.base.models.ir_actions_report: wkhtmltopdf: libpng warning: iCCP: known incorrect sRGB profile

2. Invoice format has problem: odoo.addons.base.models.ir_actions_report: wkhtmltopdf: libpng warning: iCCP: known incorrect sRGB profile
Exit with code 1 due to network error: ContentNotFoundError

I have noticed whenever ContentNotFoundError. It means the problem with the invoice format.

Here is the screenshot of the invoice


Note: I have updated the report.url in system parameters.