Unable to acquire a Read connection

There is a problem with task “Get User Group” in the process . ERROR:An error occurred while evaluating expression: UserGroup:getgroupbyname(pv!UserGroupName) (Expression evaluation error at function 'getgroupbyname': Error evaluating function 'getgroupbyname' : Unable to acquire a Read connection. Safe to retry.) (Data Outputs)

-  No process engine lock, No fatal gateway

- No multiple instances at the same time. No heavy load on the engine

-  The process model is run by the designer. No privilege issue.

Is that possible when the process is deleting group B, C... from the system, then all other group is get locked until the delete group process completed? 

Is anyone experiences that kind of behavior Group deletion lock all groups.

Thanks