Hi Rana, 
thanks for your help, it is really appreciated.
In general your code does work but I can't get it to work with my <apex:inputCheckbox> elements.
For those elements the attribute type name is not supported so I gave them the id="rowSelectCheckBox".
But the javascript seems like it can't find the checkboxes or something else won't work.
You can find attached my whole code as it is right now. It would be really nice if you could work off of that and help me get this thing to work!

Thanks from germany :)

<apex:page standardController="Rahmenvertrag_OptIn__c" recordSetVar="rv" standardStylesheets="true">
    <style>
        html .brandQuaternaryBgr{
    		background-color: white !important;
		}
        @font-face {
        	font-family: 'Delivery';
        	src: url(/resource/Delivery_W_Rg);
        }
        
        @font-face {
        	font-family: 'Delivery Headline';
        	src: url(/resource/Delivery_CdBlk);
        }
        
        .form1{
            background-color: blue;
            font-size: 28;
        }
        
        h1{
            font-family: "Delivery Headline";
            font-size: 28px;
		}  
        
        .container1 {
            background-color: white;    
        	height: 1000px;
            width: 100%;
        }
        
        .textbody {
        	font-family: "Delivery";
        	font-size: 14px;
        }
        
        .tableheader {
        	font-family: "Delivery Headline";
        }
        
    </style>
    <script>
    	function CheckUncheckAll(){
            var selectAllCheckbox=document.getElementById("checkUncheckAll");
            if(selectAllCheckbox.checked==true){
                var checkboxes =  document.getElementsById("rowSelectCheckBox");
                for(var i=0, n=checkboxes.length;i<n;i++) {
                    checkboxes[i].checked = true;
               		}
            	}	else {
                var checkboxes =  document.getElementsById("rowSelectCheckBox");
                for(var i=0, n=checkboxes.length;i<n;i++) {
                    checkboxes[i].checked = false;
                	}
            	}
            }

            var flag = false;
            function CheckUncheckAll2(){
                var checkboxes =  document.getElementsById("rowSelectCheckBox");
                if(flag == false ){
                    for(var i=0, n=checkboxes.length;i<n;i++) {
                        checkboxes[i].checked = true;
                        flag = true;
                    	}
                	} else {
                    for(var i=0, n=checkboxes.length;i<n;i++) {
                        checkboxes[i].checked = false;
                        flag = false;
                    	}
                	}
            	}
    </script>
    
    <div class="container1">
        <apex:pageBlock >
        <h1>Rahmenvertrag Teilnehmer-Liste</h1>
        <apex:form styleClass="textbody" id="form1">
        	
            <p>
                UserID: {!$User.Id}
                User Mail: {!$User.Email}
            </p>
            
            <apex:pageBlockTable value="{! rv }" var="rvt" id="table1">
                
                <apex:column value="{! rvt.KD_TN_EKP__c}" styleClass="textbody" headerClass="tableheader" headerValue="EKP" />
                <apex:column value="{! rvt.KD_TN_Name__c}" styleClass="textbody" headerClass="tableheader" headerValue="Rahmenvertrag Teilnehmer"/>
                <apex:column value="{! rvt.KD_TN_Adresse__c}" styleClass="textbody" headerClass="tableheader" headerValue="Adresse"/>
                <apex:column>
                    <apex:facet name="header"><span class="tableheader">Einwilligung<br/>Datenaustausch</span></apex:facet>
                    <apex:inputCheckbox value="{! rvt.Datenaustausch__c}" styleClass="textbody" />
                </apex:column>
                <apex:column >
                    <apex:facet name="header"><span class="tableheader">Einwilligung<br/>Werbliche Ansprache</span></apex:facet>
                    <apex:inputCheckbox value="{! rvt.Werbeansprache__c}" styleClass="textbody" id="rowSelectCheckBox"/>
                </apex:column>
                </apex:pageBlockTable>
            
            
            
            <br/>
            <table>
                <tr>
                <td style="padding-right: 20px;"><apex:commandButton action="{!save}" value="Zwischenspeichern"/></td>
                <td style="padding-right: 20px;"><apex:commandButton action="{!save}" value="Änderungen bestätigen" /></td>
                <td style="padding-right: 20px;"><input type="button" id="checkUncheckAll" value="Click Me" onClick="CheckUncheckAll2()"/></td>
				</tr>                    
            </table>
        </apex:form>
    </apex:pageBlock>
        </div>
</apex:page>