title: Keycloak Uncaught server error: org.keycloak.models.ModelException
body: <p>I work with Keycloak in version 15.0.2 and I am facing below issue due to use request:</p>
  <pre><code>http://127.0.0.1:80/auth/admin/realms/test/clients/**client_id**/user-sessions?max=100000
  </code></pre>
  <p>the issue occurred not always but 3/5 attempts.
  I tried to find any solution on the Internet but without success.</p>
  <pre><code>ERROR [org.keycloak.services.error.KeycloakErrorHandler] (default task-12885) Uncaught server error: com.fasterxml.jackson.databind.JsonMappingException: org.keycloak.models.ModelException: java.util.ConcurrentMod
  ificationException
          at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._wrapAsIOE(DefaultSerializerProvider.java:509)
          at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:482)
          at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:400)
          at com.fasterxml.jackson.databind.ObjectWriter$Prefetch.serialize(ObjectWriter.java:1510)
          at com.fasterxml.jackson.databind.ObjectWriter.writeValue(ObjectWriter.java:1006)
          at org.jboss.resteasy.plugins.providers.jackson.ResteasyJackson2Provider.writeTo(ResteasyJackson2Provider.java:328)
          at org.jboss.resteasy.core.interception.AbstractWriterInterceptorContext.writeTo(AbstractWriterInterceptorContext.java:137)
          at org.jboss.resteasy.core.interception.ServerWriterInterceptorContext.writeTo(ServerWriterInterceptorContext.java:61)
          at org.jboss.resteasy.core.interception.AbstractWriterInterceptorContext.proceed(AbstractWriterInterceptorContext.java:124)
          at org.jboss.resteasy.security.doseta.DigitalSigningInterceptor.aroundWriteTo(DigitalSigningInterceptor.java:147)
          at org.jboss.resteasy.core.interception.AbstractWriterInterceptorContext.proceed(AbstractWriterInterceptorContext.java:129)
          at org.jboss.resteasy.core.ServerResponseWriter.lambda$writeNomapResponse$2(ServerResponseWriter.java:151)
          at org.jboss.resteasy.core.interception.ContainerResponseContextImpl.filter(ContainerResponseContextImpl.java:399)
          at org.jboss.resteasy.core.ServerResponseWriter.executeFilters(ServerResponseWriter.java:219)
  </code></pre>