Log window should not be empty

SourceForge Bug Report
Sometimes, "view log" (or click on state of job for example) doesn't work: a new windows is open but displays nothing (white windows). Usually its works again after several minutes (without logout or particular actions). When the log isn't displayed in JOC Cockpit it is always displayed in old web joc. This issue apears independently of state of the task / Job (for job running or completed ...).
I have this problem on Chrome Version 59.0.3071.115 (Official Build) (64-bit) and Firefox 41.0.2.