IIS URL Rewrite 2.1 skip files that already exist - iis

Question

I have a URL rewrite in my web.config. The rewrite directives are intended to do two things:
If the URL refers to an actual file (such as a css file or image) don't rewrite
If the URL does not refer to an actual file, rewrite to index.php?request={R:1}
Case 2 works perfectly. However, if the requested file exists, I get a generic IIS response indicating an error: HTTP Error 500.50 - URL Rewrite Module Error. - and no other details. The error codes just indicate a generic rewrite module error.
What have I done wrong? This is IIS 10.0
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
<rewrite>
<rules>
<rule name="Do not rewrite existing files and folders" enabled="true" stopProcessing="true">
<match url="^(.*)$" />
<conditions logicalGrouping="MatchAll" trackAllCaptures="false">
<add input="{REQUEST_FILENAME}" matchType="IsFile" />
</conditions>
<action type="None" url="{R:0}" appendQueryString="true" logRewrittenUrl="true" />
</rule>
<rule name="Framework Parsing" stopProcessing="true">
<match url="^(.*)$" />
<conditions logicalGrouping="MatchAll" trackAllCaptures="false" />
<action type="Rewrite" url="index.php?request={R:1}" logRewrittenUrl="true" />
</rule>
</rules>
</rewrite>
<caching>
<profiles>
<remove extension=".php" />
</profiles>
</caching>
<httpProtocol>
<customHeaders>
<remove name="X-Powered-By" />
<add name="Access-Control-Allow-Origin" value="*" />
<add name="X-Content-Type-Options" value="nosniff" />
<add name="X-Xss-Protection" value="1; mode=block" />
<add name="X-Frame-Options" value="SAMEORIGIN" />
<add name="Referrer-Policy" value="origin" />
</customHeaders>
</httpProtocol>
<!-- staticContent>
<clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="8.00:00:00" setEtag="true" />
</staticContent -->
</system.webServer>
</configuration>
This is the error page details:
Module RewriteModule
Notification BeginRequest
Handler StaticFile
Error Code 0x80070005
Requested URL XXXXXXXXX/css/foundation/foundation.min.css
Physical Path XXXXXXXXX\public\css\foundation\foundation.min.css
Logon Method Not yet determined
Logon User Not yet determined

Answer 1

I notice that its login method and user is not determined.
Please try to enable anonymous authentication for your rewrite rule. And ensure IUSR have permission to access these files.