Hello,
The screenshot you provided display only tasks for the same project. Is it the only project analyzed on this SonarQube instance?
If not, maybe the task of another project is stuck and blocks all the other tasks. Try and find it and have a look at this task's logs.
If so, there is no obvious reason that comes to mind.
In both cases, have you tried restarting SQ ?
Finally, you should upgrade to 5.6, it's the current Long Time Support (LTS) version of SQ. You won't get much support (if at all) for prior versions and anyway, background task processing can only be more reliable in more recent versions.
Cheers,