STM32 flash download failed - Target dll has been cancelled

Hello, I am using Keil uvision 4 (arm-mdk453 package) for STM32VL discovery board. It used to be working properly, but now I'm having a problem when I try to download to flash or to debug. I receive popup messages like ''Internal command error'', ''Flash download failed: target dll has been cancelled''. I did upgrade to (mdk454 package) but still have the same problem. Any help will be highly appreciated.

Best Regards,

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