It is not a known issue and without more details it is hard to determine its source. Please compare your reports and check what is different between successfully and unsuccessfully exported reports.
If you need further help, please elaborate on your project type, structure, used product version, conditions under which the export fails, browsers. Any additional details from debugging will be helpful. If you have some code to attach, please feel free to open a support ticket.
the Telerik team
Have you tried the new visualization options in Telerik Reporting Q1 2013? You can get them from your account.