I also got this bug, but in my case it was in the second restart. I have Ubuntu Mini 14.04 "only the base system" + LXQT Repos in Virtualbox. I upgraded with "dist-update" to the latest and i was unable to add new Quick Launch Icons. However, i had before a Firefox icon and it worked fine.
After a second restart with no new update/upgrade done, the Firefox icon had disappeared and the same bug for adding a new Quick Launch icon appeared. I had the base LXQT 0.7 from the beginning and it was the first time upgrading with "dist-update" when the Qt5 support was announced.
Hope this helps,