What part of the application are you using when the log out occurs ? And does it occur even if you are still actively using the application ?
I'm using version 5.22 'out of the box', and I haven't noticed that problem when in either Activiti Explorer or Modeller. I can leave it a reasonable time and I'm still logged in when I come back to it.
I'm using the community edition of Activiti which is V5.22, I think that is the version which is used in the commercial APS version 1.8.1 as well, but not sure how different some of the components are. So my experience may not be relevant, but if you are still active it doesn't sound like a time-out, do you see any errors or other information at the time it logs you out - and is the engine still alive ?