'killed by server' usually means that it is taking far too long to compile or execute. Since it compiled successfully, it looks like an execution problem. My concern is that check50 is reporting fails on all file creations. That could be an infinite loop. It could also mean that you're opening all the files but not closing them, or waiting till the end to close them all. Having all output files open at once has also been known to cause issues. There could be any number of reasons for the problem, but without seeing any code, it's just guesswork here.
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