From a Crowd support case:

"I have a given a regular expression for the directory where all these users are imported. Now after integration with crowd When I try to reset the password of a confluence user in confluence with invalid conditions( not according to regular expression given), Then I get the following exception with confluence System error page being displayed."

Cause:
com.atlassian.user.EntityException: Fault: com.atlassian.crowd.integration.exception.InvalidCredentialException: Fault: com.atlassian.crowd.integration.exception.InvalidCredentialException
at com.atlassian.crowd.integration.atlassianuser.CrowdUserManager.alterPassword(CrowdUserManager.java:170)

caused by: com.atlassian.crowd.integration.exception.InvalidCredentialException: Fault: com.atlassian.crowd.integration.exception.InvalidCredentialException: Fault: com.atlassian.crowd.integration.exception.InvalidCredentialException
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)

caused by: org.codehaus.xfire.fault.XFireFault: Fault: com.atlassian.crowd.integration.exception.InvalidCredentialException
at org.codehaus.xfire.fault.Soap11FaultSerializer.readMessage(Soap11FaultSerializer.java:31)

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Suggested fix:

1. Add a new subclass of EntityException called InvalidPasswordException
2. Throw InvalidPasswordException from CrowdUserManager.alterPassword()
3. Modify bucket's DefaultUserAccessor to rethrow InvalidPasswordException
4. Catch InvalidPasswordException in Confluence's ChangeUserPasswordAction and display a meaningful error message to the user.