[jtp10181]

the file natives are in the core so it does not matter what mod you are running. is this on linux or windows?

I looked in the source and this should be where its throwing the runtime error.

	// adding a new line in a middle of already existing file
	FILE* pTemp;
	char buffor[2048];

	if ((pTemp = tmpfile()) == NULL)
	{
		amx_RaiseError(amx, AMX_ERR_NATIVE);
		return 0;
	}

it is creating a temp file. Does the user your server runs as have permission to write to the systems temp directory? I Bet thats the problem.
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Anyway, once amxx 1.75 comes out I think there will be an "fputs" native which should fix the problem with the new file native code.