I created the subreports also with Jaspersoft studio, the main report contains these 3 subreports. Below is part of the mainreport:
<detail>
<band height="165" splitType="Stretch">
<staticText>
<reportElement x="0" y="4" width="100" height="15" forecolor="#364AB3">
<property name="com.jaspersoft.studio.unit.x" value="pixel"/>
<property name="com.jaspersoft.studio.unit.y" value="pixel"/>
<property name="com.jaspersoft.studio.unit.height" value="pixel"/>
</reportElement>
<textElement lineSpacing="Single">
<font fontName="Arial" size="12" isBold="true"/>
</textElement>
<text><![CDATA[Hosting Servers]]></text>
</staticText>
<subreport>
<reportElement x="0" y="21" width="555" height="30">
<property name="com.jaspersoft.studio.unit.x" value="pixel"/>
<property name="com.jaspersoft.studio.unit.y" value="pixel"/>
<property name="com.jaspersoft.studio.unit.width" value="pixel"/>
<property name="com.jaspersoft.studio.unit.height" value="pixel"/>
</reportElement>
<connectionExpression><![CDATA[$P{REPORT_CONNECTION}]]></connectionExpression>
<subreportExpression class="java.lang.String"><![CDATA["Subreport Overview Hosting Servers.jasper"]]></subreportExpression>
</subreport>
<staticText>
<reportElement positionType="Float" x="0" y="61" width="100" height="15" forecolor="#364AB3">
<property name="com.jaspersoft.studio.unit.x" value="pixel"/>
<property name="com.jaspersoft.studio.unit.y" value="pixel"/>
<property name="com.jaspersoft.studio.unit.height" value="pixel"/>
</reportElement>
<textElement lineSpacing="Single">
<font fontName="Arial" size="12" isBold="true"/>
</textElement>
<text><![CDATA[Standard Servers]]></text>
</staticText>
<subreport>
<reportElement positionType="Float" x="0" y="78" width="555" height="30">
<property name="com.jaspersoft.studio.unit.x" value="pixel"/>
<property name="com.jaspersoft.studio.unit.width" value="pixel"/>
<property name="com.jaspersoft.studio.unit.height" value="pixel"/>
<property name="com.jaspersoft.studio.unit.y" value="pixel"/>
</reportElement>
<connectionExpression><![CDATA[$P{REPORT_CONNECTION}]]></connectionExpression>
<subreportExpression class="java.lang.String"><![CDATA["Subreport Overview Standard Servers.jasper"]]></subreportExpression>
</subreport>
<staticText>
<reportElement positionType="Float" x="0" y="118" width="100" height="15" forecolor="#364AB3">
<property name="com.jaspersoft.studio.unit.x" value="pixel"/>
<property name="com.jaspersoft.studio.unit.y" value="pixel"/>
<property name="com.jaspersoft.studio.unit.height" value="pixel"/>
</reportElement>
<textElement lineSpacing="Single">
<font fontName="Arial" size="12" isBold="true"/>
</textElement>
<text><![CDATA[Virtual Servers]]></text>
</staticText>
<subreport>
<reportElement key="" positionType="Float" x="0" y="135" width="555" height="30" isRemoveLineWhenBlank="true" isPrintInFirstWholeBand="true" isPrintWhenDetailOverflows="true">
<property name="com.jaspersoft.studio.unit.x" value="pixel"/>
<property name="com.jaspersoft.studio.unit.y" value="pixel"/>
<property name="com.jaspersoft.studio.unit.width" value="pixel"/>
<property name="com.jaspersoft.studio.unit.height" value="pixel"/>
</reportElement>
<connectionExpression><![CDATA[$P{REPORT_CONNECTION}]]></connectionExpression>
<subreportExpression class="java.lang.String"><![CDATA["Subreport Overview Virtual Servers.jasper"]]></subreportExpression>
</subreport>
</band>
</detail>
However, the problem isn't the working of the report, but the failing upload. I have created more reports in the past with one or more subreports without any problem uploading and running them.