To solve this i deleted this part out of the web.config
Worked for me on IIS 10, osTicket 1.15
<rewrite>
<rules>
<rule name="HTTP api" stopProcessing="true">
<match url=".*/?api/(.)$" ignoreCase="true"/>
<conditions>
<add input="{REQUEST_FILENAME}" matchType="IsFile"
ignoreCase="false" negate="true" />
<add input="{REQUEST_FILENAME}" matchType="IsDirectory"
ignoreCase="false" negate="true" />
</conditions>
<action type="Rewrite" url="{R:1}api/http.php/{R:2}"/>
</rule>
<rule name="Site pages" stopProcessing="true">
<match url=".*/?pages/(.)$" ignoreCase="true"/>
<conditions>
<add input="{REQUEST_FILENAME}" matchType="IsFile"
ignoreCase="false" negate="true" />
<add input="{REQUEST_FILENAME}" matchType="IsDirectory"
ignoreCase="false" negate="true" />
</conditions>
<action type="Rewrite" url="{R:1}pages/index.php/{R:2}"/>
</rule>
<rule name="Staff applications" stopProcessing="true">
<match url=".*/?scp/apps/(.*)$" ignoreCase="true"/>
<conditions>
<add input="{REQUEST_FILENAME}" matchType="IsFile"
ignoreCase="false" negate="true" />
<add input="{REQUEST_FILENAME}" matchType="IsDirectory"
ignoreCase="false" negate="true" />
</conditions>
<action type="Rewrite" url="{R:1}scp/apps/dispatcher.php/{R:2}"/>
</rule>
</rules>
</rewrite>