From Aniceto Goñi:

We have just upgraded from a 3.13.1 jira with MySQL and Kaamelot to a 3.13.5 jira with oracle and minyaa (version 3.13.5.1.1). From that moment, we have lost the sub-task list on all the issues. Task are still on the system, you can search them and access them, and they even have a correct parent issue, it's just we don't get them listed on the issue. But, if we click on the 'Printable' version of the issue, we get to see the sub-task list.

Seems like the installation has modified some .jsp somewhere, and the issue page is not displayed correctly.