Portal backend issues after migrating to new version

Hello, We have problem with the Portal application running on Axon Ivy Engine 7.0.12 with Postgres 9.6 as a System Database. The first 2-3 tasks we execute are running smoothly. After that the performance is considerably deteriorated. If we try to go to Cases, the Html Dialog is not loaded at all. Or if I go to Tasks, no tasks are loaded. I will appreciate if anyone could hava a look at the below errors and give us a clue. Thanks,

AA

The errors that I could see in the error logs are:

07:05:15.176 ERROR [runtimelog.Test.PortalKit.web_service] [ivy background operation pool-thread-4] [request=HTTP GET call(6521.6520.89.0), session=30 (demo), task=6520, application=2, requestId=180, executionContext=SYSTEM, pmv=Test$PortalKit$1, processElement=14E8BC51CC56193A-f120] 
Web Service Element: Call to http://localhost:8080/ivy/ws/System/PortalConnector/1380566F9095B9C4 (Operation[name=analyzeExpiryStatistic,port=TaskServicePort,parameters=[TypedMapping /name=arg0.analyzeExpiryStatistic.apps /type=Array<String> /expression=in.applicationNames, TypedMapping /name=arg0.analyzeExpiryStatistic.jsonQuery /type=String /expression=in.jsonQuery, TypedMapping /name=arg0.analyzeExpiryStatistic.userName /type=String /expression=in.taskStatisticRequester],parameterMappingOptions=RequestParameterMappingOptions[mapNullValues=false,autoInitializeFirstLevelFields=true]]) failed !
07:05:15.178 ERROR [runtimelog.Test.PortalKit.user_code] [http-nio-8080-exec-2] [request=HTTP GET call(6521.6520.89.2), session=30 (demo), task=6520, application=2, requestId=180, executionContext=30 (demo), pmv=Test$PortalKit$1, hd=ch.ivy.addon.portalkit.component.StatisticWidget, processElement=14E8BC51CC56193A-f7]