Admin expandable menu

When I try to see the submenu in admin panel, nothing happens.

Debugging I found that moodle POSTs :

1. elementid:
   expandable_branch_71_siteadministration
2. id:
   siteadministration
3. type:
   71
4. sesskey:
   vZF7QtpcDt
5. instance:
   5

And in the apache log I find:  Cannot obtain session lock for sid: mregq937h9vtfd7rhkokjjdpd4

I use 2.8.1+ moodle and I don't have a clue about what could It be.