Problem:

My Search Console tells me, that this is producing crawling-errors:
<script>var et_site_url='
https://site.com';var et_post_id=313;function et_core_page_resource_fallback(a,b){"undefined"===typeof b&&(b=a.sheet.cssRules&&0===a.sheet.cssRules.length);b&&(a.onerror=null,a.onload=null,a.href?a.href=et_site_url+"/?et_core_page_resource="+a.id+et_post_id:a.src&&(a.src=et_site_url+"/?et_core_page_resource="+a.id+et_post_id))}
</script>

Why are you seeing this?

This is because you have enabled the Static CSS File Generation option in Dashboard => Divi => Theme Options => Builder => Advanced. This option generates a CSS file to store all of the Divi custom styles which reduces the server requests and give you extra points on those speed test services like gtmetrix.
The problem now is Google trying to index this file which has no useful content, and to prevent this, we use a Javascipt code that adds a 404 for the page.

Is this bad for SEO?

Absolutely not, here's an answer by John Mueller - webmaster trends analyst at Google- which you can find in this link

3) What about the funky URLs that are “clearly broken?” When our algorithms like your site, they may try to find more great content on it, for example by trying to discover new URLs in JavaScript. If we try those “URLs” and find a 404, that’s great and expected. We just don’t want to miss anything important (insert overly-attached Googlebot meme here). http://support.google.com/webmasters/bin/answer.py?answer=1154698

That's exactly your case with the resources callback URL, and so.. this will cause no harm, whatsoever to your SEO ranking and it will help to prevent indexing the CSS file pages.

I really want to fix this error regardless what was mentioned above

In this case, we suggest that you disable the Static CSS File Generation option in Dashboard => Divi => Theme Options => Builder => Advanced, And use other services for CSS caching and minifying. This article from our blog should help.

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