Re: Facing issue to retrieve task list using createTaskQuery().taskCandidateUser at Activiti 6.0.0
Upgrading from Activiti 5 to 6 is not possible because Activiti 6 is introducing new ACL concept around workflows.
I would suggest to migrate your existent definitions, instances and extensions to Activiti 6 starting from a vanilla installation. For this purpose you may want to use the Activiti SDK project available here:
This SDK is based on Activiti Community 6 and it allows you to manage your Activiti extensions building your JAR, the overlayed Activiti 6 WAR and the related Docker image. The SDK will also deploy locally the entire platform using Docker.
I know that if you try to upgrade Activiti from 5 to 6 the engine executes all the needed patches but the problem is related to your current data that is not correctly migrated. In order to solve, you should adjust by yourself the data with specific SQL statements related to your specific changes.