Any additional information in the SUM\abap\trc\MEMORY.trc log file?
Stop the SUM tool completely.
Change the property com.sap.sdt.severity to DEBUG in these files
-SUM\sdt\param\logging.properties
-SUM\sdt\prop\logging.properties
-SUM\java\param\logging.properties
Restart SUM and check the trace files.
Regards
RB