We have not been able to reproduce this behavior. Can you clarify what excel export are you using: Excel 97- 2003 which is binary format or the Excel 2007 (Open XML) format? Can you test exporting from our online demos
and see if you have such problems? If the excel files from our demo open correctly, but the issue occurs with all reports from your server, then please check whether you're using any compression modules on your server and whether the issue can be still reproduced after disabling them.
On a side note, searching about this problem we found reports that it recently occurred after applying a Windows security patch. We found information that this update is causing problems with Excel files in general. Please, check the following thread: Excel Security Update Causing Issues with Excel (KB2597166)
the Telerik team
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