best joomla cache settings - to solve online status issue?

For some time now I have had a problem with the status display of users.
Although they are online, "absent" is displayed. A click on "online" brings a hint that the status has been saved successfully - but nothing changes.
Currently I have
cache handler: File
Cache Time: 15
System Cache: ON - Progressice caching

Session Settings
Session Handler: PHP
Session Lifetime: 30
shared sessions: No

Do you have any idea how to solve this problem?

thank you
Tom