How to find UI errors

Hi,

we get following error if we save & close one of our user definied entities. The details doesn't help to find the error. Is there a best practice to find those errors?

Here's the data information of the message:

Microsoft Dynamics CRM Error Report Contents

<CrmScriptErrorReport>  

<ReportVersion>1.0</ReportVersion>

<ScriptErrorDetails>   

<Message>TypeError: a is undefined</Message>   

<Line>5</Line>   

<URL>/_common/global.ashx?ver=-1065975225</URL>   

<PageURL>/main.aspx?etc=10005&extraqs=%3f_CreateFromId%3d%257bC61E8BE1-63EA-E411-80C5-00155DF5A224%257d%26_CreateFromType%3d112%26_gridType%3d10005%26etc%3d10005%26id%3d%257bC81E8BE1-63EA-E411-80C5-00155DF5A224%257d%26rskey%3d810067873&pagemode=iframe&pagetype=entityrecord&rskey=810067873

</PageURL>   

<Function>anonymousr:aisundefined</Function>   

<CallStack>    

<Function>anonymousr:aisundefined</Function>   

</CallStack>  

</ScriptErrorDetails>  

<ClientInformation>   

<BrowserUserAgent>Mozilla/5.0 (Windows NT 6.1; rv:37.0) Gecko/20100101 Firefox/37.0

</BrowserUserAgent>   

<BrowserLanguage>undefined</BrowserLanguage>  

  <SystemLanguage>undefined</SystemLanguage>   

<UserLanguage>undefined</UserLanguage>   

<ScreenResolution>1680x1050</ScreenResolution>   

<ClientName>Web</ClientName>   

<ClientTime>2015-04-24T13:54:51</ClientTime>  

</ClientInformation>  

<ServerInformation>    

<OrgLanguage>1031</OrgLanguage>    

<OrgCulture>1031</OrgCulture>    

<UserLanguage>1033</UserLanguage>    

<UserCulture>1031</UserCulture>    

<OrgID>{0AA115C0-A158-E111-9D83-7AE391F5378C}</OrgID>    

<UserID>{58EE542B-4F19-E411-9F46-7AE391F5378C}</UserID>    

<CRMVersion>6.0.3.106</CRMVersion>  

</ServerInformation>

</CrmScriptErrorReport>

Regards

Marco