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Reverting from jdk-11.0.4 to jdk1.8.0_221 fixed this problem. Possible symptoms of this problem may have been previously reported in opening two instances of Oracle SQL Developer software doesn't allow using DBA Console
This problem also occurs when using jdk-12.0.2: only one system_cache folder after starting up two instances of SQL Developer.
To make matters worse, after closing the two instances, starting up a single instance fails consistently with