We’ve also encountered this issue. Our Spring Boot app fails to start up with this error (entity class name redacted:

Caused by: org.hibernate.MappingException: Could not instantiate id generator [entity-name=<entity class>]
	at org.hibernate.id.factory.internal.DefaultIdentifierGeneratorFactory.createIdentifierGenerator(DefaultIdentifierGeneratorFactory.java:124) ~[hibernate-core-5.4.9.Final.jar:5.4.9.Final]
	at org.hibernate.mapping.SimpleValue.createIdentifierGenerator(SimpleValue.java:344) ~[hibernate-core-5.4.9.Final.jar:5.4.9.Final]

hibernate-core: 5.4.9.Final
JVM: Zulu 8.42.0.23-CA-macosx
OS: Macos 10.14.6
DB: Oracle 12c

I did some debugging with IntelliJ and traced things to the getSequenceIncrementValue() in SequenceStyleGenerator like Florian reported.

Reverting to hibernate-core to 5.3.12.Final avoided the error and our app started up and worked with the Oracle sequence set to increment by 50 to match the default allocationSize of 50 for the @SequenceGenerator annotation.