After updating to Moodle 1.9.9 I found out that I lose ability to delete quiz attempts (even as admin,tested on different browsers).

1. Select quiz report, then use group combobox to choose attempts from one specific group
2. Mark an attempt on quiz report
3. Choose "delete selected attempts", than "yes"
4. Look at reloaded report - the attempt are intact

It will work normally if you select "all groups" mode.
Apparently group mode selection combo box interferes somehow with new session check.