No graphs on web analytics

Hello,

we have trouble with not shown graphs on the web analytics. All graphs windows show "Invalid image data. Please check eventlog for more details."

On the eventlog the following error is shown:

EventID: 32936

EventType: E

EventTime: 10/14/2014 11:33:35 AM

Source: Get report graph

EventCode: E

UserName: administrator

IPAddress: 10.255.0.20

EventDescription: Message: Method not found: 'Void System.Web.UI.DataVisualization.Charting.Chart.set_RightToLeft(System.Windows.Forms.RightToLeft)'. 
Stack Trace: 
   at CMS.Reporting.ReportGraph.CreateChart(ReportGraphInfo graphInfo, DataSet dsGraphData, ContextResolver resolver, Boolean returnByteData)
   at CMS.Reporting.ReportGraph.CreateChart(ReportGraphInfo graphInfo, DataSet dsGraphData, ContextResolver resolver)
   at CMSModules_Reporting_Controls_ReportGraph.GetReportGraph(ReportGraphInfo reportGraph)


EventUrl: /CMSModules/WebAnalytics/Tools/Analytics_Report.aspx?statCodeName=landingpage&dataCodeName=landingpage&reportCodeName=landingpage.yearreport%3blandingpage.monthreport%3blandingpage.weekreport%3blandingpage.dayreport%3blandingpage.hourreport&multilingualreportcodename=landingpage.multilingual.yearreport%3blandingpage.multilingual.monthreport%3blandingpage.multilingual.weekreport%3blandingpage.multilingual.dayreport%3blandingpage.multilingual.hourreport&displaytitle=0

EventMachineName: RZW-WEB-ASP

EventUserAgent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; WOW64; Trident/7.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; MAAR; InfoPath.3; .NET4.0C; .NET4.0E)

EventUrlReferrer: http://www.vechta.de/CMSModules/WebAnalytics/Tools/Analytics_Report.aspx?statCodeName=landingpage&dataCodeName=landingpage&reportCodeName=landingpage.yearreport;landingpage.monthreport;landingpage.weekreport;landingpage.dayreport;landingpage.hourreport&multilingualreportcodename=landingpage.multilingual.yearreport;landingpage.multilingual.monthreport;landingpage.multilingual.weekreport;landingpage.multilingual.dayreport;landingpage.multilingual.hourreport&displaytitle=0

Data for example pageviews are logged and shown in the table view under the missing graphs.

Best regards