IIS Reverse Proxy for Nextcloud - preserve HTTP_HOST

hi,

I'm trying to reverse proxy cloud.mydomain.com to a debian nextcloud vm internally reachable under cloud.mydomain.local. The website itself works fine, after telling nextcloud not to use gzip for compression, but I can't use nextclouds sync client. After entering the external url, the sync client tries to connect to the internally used URL which is configured in url rewrite. I guess this is because it is transferred by HTTP_HOST.

Can I somehow make nextcloud think the connection came from a different URL?

This is my web.config so far: (I tried to overwrite HTTP_HOST and SERVER_NAME with the desired external url, but it does not work. I'm now testing with little php file just echoing HTTP_HOST and SERVER_NAME)

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
<rewrite>
<outboundRules>
<rule name="ReverseProxyOutboundRule1" preCondition="ResponseIsHtml1">
<match filterByTags="A, Form, Img" pattern="^http(s)?://cloud.mydomain.local/(.*)" />
<action type="Rewrite" value="http{R:1}://cloud.mydomain.com/{R:2}" />
</rule>
<rule name="RestoreAcceptEncoding" preCondition="NeedsRestoringAcceptEncoding">
<match serverVariable="HTTP_ACCEPT_ENCODING" pattern="^(.*)" />
<action type="Rewrite" value="{HTTP_X_ORIGINAL_ACCEPT_ENCODING}" />
</rule>
<rule name="RestoreHttpHost" preCondition="NeedsRestoringOriginalHost">
<match serverVariable="HTTP_HOST" pattern="^(.*)" />
<action type="Rewrite" value="{HTTP_X_ORIGINAL_HOST}" />
</rule>
<preConditions>
<preCondition name="ResponseIsHtml1">
<add input="{RESPONSE_CONTENT_TYPE}" pattern="^text/html" />
</preCondition>
<preCondition name="NeedsRestoringAcceptEncoding">
<add input="{HTTP_X_ORIGINAL_ACCEPT_ENCODING}" pattern=".+" />
</preCondition>
<preCondition name="NeedsRestoringOriginalHost">
<add input="{HTTP_X_ORIGINAL_HOST}" pattern=".+" />
</preCondition>
</preConditions>
</outboundRules>
<rules>
<rule name="ReverseProxyInboundRule1" stopProcessing="true">
<match url="(.*)" />
<action type="Rewrite" url="http://cloud.mydomain.local/{R:1}" />
<serverVariables>
<set name="HTTP_X_ORIGINAL_ACCEPT_ENCODING" value="{HTTP_ACCEPT_ENCODING}" />
<set name="HTTP_ACCEPT_ENCODING" value="" />
<set name="HTTP_HOST" value="cloud.mydomain.com" />
<set name="SERVER_NAME" value="cloud.mydomain.com" />
</serverVariables>
</rule>
</rules>
</rewrite>
<httpErrors errorMode="Detailed" />
<httpProtocol>
<customHeaders>
<remove name="X-Powered-By" />
</customHeaders>
</httpProtocol>
</system.webServer>
</configuration>

Thanks in advance,

R'N'L