EDIT

... this is related to hibernate upgrade to v5.4 (#10067)

This happens when deserializing a org.hibernate.proxy.pojo.bytebuddy.SerializableProxy during ehCache read from disc.

Generally hibernate proxies (org.hibernate.proxy.HibernateProxy) are serialized by using the writeReplace() method which transforms the HibernateProxy into a SerializableProxy. For deserializing the readResolve() method from SerializableProxy is used.

This works for serializing/deserializing during httpInvoker transportation. It also seems to work with the javassist SerializableProxy (org.hibernate.proxy.pojo.javassist.SerializableProxy). But does not work for the bytebuddy version.

The problem occurs during deserialization of bytebuddy.SerializableProxy itself (so still before calling readResolve()). The exact moment when it happens is deserialization of the parameter "Class[] SerializableProxy.identifierSetterMethodParams" which has as content "int" and the net.sf.ehcache.util.PreferredLoaderObjectInputStream used by ehCache uses Class.forName(String name, boolean initialize, ClassLoader loader) for class loading which does not handle primitive types ("this method cannot be used to obtain any of the Class objects representing primitive types").
httpInvoker uses org.springframework.remoting.rmi.CodebaseAwareObjectInputStream.
TODO: check what method spring uses for class loading.

javassist:

It is unclear why we did not observe this problem with javassist under hibernate 5.1. as the javassist.SerializableProxy looks equal in terms of datastructure to be loaded (https://docs.jboss.org/hibernate/orm/5.1/javadocs/org/hibernate/proxy/pojo/javassist/SerializableProxy.html).