Regardless of who or what actually caused it, the error message is generated by your website code. We are not experts on all software you could possibly host with us.
A quick Google search pointed me to this topic on the WordPress forum, which suggests that this message is coming from the WooCommerce plugin for WordPress:
This article from WooCommerce has more details about the thumbnail generation:
One part of this documentation worries me, which is this section:
This is because WP Background Processing relies on the WordPress HTTP API, which requires you to attach your BasicAuth credentials to requests.
It might be that this functionality is blocked by this security system:
If so, I’m not sure how this issue can be solved.
But to be clear, I’m not an expert on WooCommerce. I’m just telling you what I found through 5 minutes of Googling combined with my knowledge of our hosting.