Refused to execute script from [URL] because its MIME type ('text/html') is not executable, and strict MIME type checking is enabled

@gouthamraon
Same issue on my side.
The site is 100% private.
I have a custom module that redirects unauthenticated users to login page.

It works like a charm when Aggregate CSS is not active
It doesn't work when Aggregate CSS is active (css is not loaded because wrong mime type, same error as yours)

Did you find a way in the meantime?

I found a way. may be not a good practice but it works!

First attempt:
In mymodule/src/EventSubscriber/Mysbsciber.php

$loginurl = Url::fromRoute('user.login')->toString(); 
$event->setResponse(new RedirectResponse($loginurl, 301));

It works only when aggregate CSS is disabled

Second attempt:
In mymodule/src/EventSubscriber/Mysbsciber.php

$loginurl = Url::fromRoute('user.login')->toString(); 
header('Location: '.$loginurl)
exit;

It works only when aggregate CSS is disabled

Third attempt:
In mymodule/src/EventSubscriber/Mysbsciber.php

$loginurl = Url::fromRoute('user.login')->toString(); 
?>
<script type="text/javascript"> window.location.rel="noopener" target="_blank" href= "<?=$loginurl?>";>
</script>
 <?php

It works in any case (css aggregate active or inactive)

I don't think this issue is coming from advagg module. We are also facing this issue and trying to figure out whats causing this but we are not using this module. Maybe try to uninstall this module once and check if the issue still persists to confirm if the issue is from this module.

Unfortunately I couln't find the reason.
Our custom module is redirecting anonymous users to login page by way of a controller (based on this example: https://drupal.stackexchange.com/a/223109)
Redirection works perfectly but style is not loaded when "aggregate css" is selected.

So we have deleted the controller and now we use a simple hook (https://drupal.stackexchange.com/a/295593) may be not the best practice but now it works!

@gilbertdelyon Probably because you alter also the response for the css request which gets the aggregated files.
You can try something like this in your EventSubscriber

if (php_sapi_name() == 'cli' || strpos($event->getRequest()->getRequestUri(), 'sites') !== FALSE) {
  return;
}

Thank you, I will try

BTW: As explained in #6 , since I had this issue I now redirect by way of a hook instead of a controller (and it works!).
If the suggestion in #7 works, I worry If I should go back to a controller.
Which wy is the most recommended: hook or controller?

@radubutco

Thank you for your precious help #7. It works!

In that case, this issue can be closed right?
Closing this. Reopen if you feel otherwise.