I will post a very specific issue here with an "easy" solution but this might not solve all possible problems with "Render Failed" issues.
The problem came up with anything localized in German and that means Windows 2003, SQL 2005, WSS and MOSS. The problem can also occur on English and any other localized systems and for that please look below for the English screen-shots.
The reported problem:
How to reproduce?
Create a new customer list.
Add one column as datetime and call it MyDate.
Create a Grouped By view and choose MyDate.
Enter e.g. three entries with the dates
14 Dec 2006
12 Dec 2008
15 Dec 2008
As you can see when you want to expand one of the problem groups DARSTELLUNGSFEHLER or in English RENDER FAILED.
How to solve or what is the Root-Cause?
Change the Standardsprache (Default Language) for ALL SharePoint relevant accounts used to access the SQL Server from German to English.
And how looks this on an English System?
And the English team site?
What is behind the curios behavior?
WSS is an application written for an international market and has also it's own code to use international features (.NET FrameWork) like Regional Settings and languages (language packs). Therefore the access to SQL must be with one "language" and that is English. The problem here is the date format you could use to write your own SELECT, UPDATE, DELETE and INSERT statements. Any other language using a different dateformat than mdy (month day year) could run into this behavior when you're using the repro steps I reported here.
When you add the domain account with the SQL-Manager to that accounts which should be used to access SQL it depends on the installed language (SQL Server) what is the default value for "Default Language". On a Spanish system it would be Spanish etc.
If you see any other Render Failed issue it MUST NOT be related to this behavior I reported here! You need to contact the known channels (NewsGroups, Communities or Microsoft Support) to help you further.