Wrong class file IOException.class

While compiling an old program in the new v6.00 I receive following error:

cannot access java.io.IOException
bad class file: java\io\IOException.class(java\io:IOException.class)
class file has wrong version 52.0, should be 50.0
Please remove or make sure it appears in the correct subdirectory of the classpath.
import java.io.IOException;

How can I solve this?

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