No jtag device found after program download (LPC2378)

Hello,

I use uVision V3.50 and the evaluation board MCB2300 (Keil) with an LPC2378.
I have compiled some of the code examples and was able to download them to flash via JTAG using ULINK adapter.
After I had downloaded a selfwritten program, I am not able to download anything anymore. I get a popup saying "No JTAG devices found".

Does anyone have any idea how this can be solved?

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