Sometimes the backpack icon gets corrupted and won't work; then you have go clear the icon and drag a new one from "Actions - Menu Actions: into the hot bar slot, and the new one should work.
The default for open inventory in EC is "i," unless it's been changed.
You can see what actions are what assigned to what keys under "Options," which is under "User Settings" - but here's the screwy part
User Settings is not a default icon on a hotbar. You have to go into "Actions" (which is a default icon) and then Menu Actions to find User Settings (looks like a wrench and screwdriver) and drag into a hot bar. Then you can open User Settings to check options
At least I've found this to be true on every character I've ever created in EC. Isn't that what everybody else sees? If so, it's a real oversight not making User Settings a default icon on the hot bar.