I got the exact same notification last night

me too today

Glad it's not just me! :-)

to exclude the theme as culprit what theme do you use? The email came to one that is based on the helix ultimate 2.1.2

I use JoomlaCK's Template Creator, but it doesn't add anything like this to my knowledge. And the fact that we use 2 different themes would lead me to think it's not the theme. I'll make a Cassi controlled dummy site and see if it faults.

I've fixed this by simply disabling the module for breadcrumbs (not something I use on sites anyway but its enabled by default)

Once disabled, request validation and they will be fixed in the meantime.

You can test validation here: https://search.google.com/test/rich-results

I manage 80+ sites. Going through and disabling the module is just a waste of time.

However, I did disable the Joomla Breadcrumbs module, and installed the Advanced Breadcrumbs module from UWIX Web Development. I started the Validation, but also went to the validation test and ran it. The site now passes without this issue. Seems it's definitely an issue with the native Breadcrumbs module.

When open on the "Modules" I get An error has occurred. 404 Component not found.

No option to disable breadcrumbs is available. Unless its hidden somewhere else?

You're going to Extensions | Site Modules and that is giving the Component Not Found? So, you can't see any of your modules? That sounds like a totally different issue to the Google issue. However, if you can get to PHPMyAdmin you could change it's published parameter to "0" to disable it. If you're not seeing any modules, though, that's not what you need.

On the Dutch Joomla forum we have also had multiple questions about this problem.
People with different templates.
I myself have also had the problem with templates made with JoomlArt T4 Framework.
I agree with trogladyte that it seems that this has to do with the native Breadcrumbs technique from the core of Joomla.
But not specifically the module, because if it is not activated, Google will find the same errors on the page.

I just wouldn't know how to solve it.
I hope there are people here who have a fix for this.

Same problem with www.deijsmannetjes.nl. Since 5 days. Only with Joomla articles, not with other pages. I am using the Helix Ultimate template.

Disabling schema.org plugin gets rid of the <script> line that annoys Google.

Issue is with the output of that list indeed:

On a test site, the Joomla output is:

"itemListElement": [
        {
            "@type": "ListItem",
            "position": 1,
            "item": {
                "@id": "https://example.com/",
                "name": "Home"
            }
        },
....
]

That's incorrect because there should be a "name" property:

"itemListElement": [
        {
            "@type": "ListItem",
            "position": 1,
            "name": "Home",
            "item": {
                "@id": "https://example.com/",
                "name": "Home"
            }
        },
....
]

Please test PR #45316. Thanks.