The message says it all. The BOXI license has expired (or is not in the system) and you can only use the administrator account to enter a new one. The only tool you can enter a new keycode is the CMC. So, go to the CMC log in administrator and look at the licenses section (right hand side). Is there a keycode shown? Then it has a date below it when it expired.
Is there no keycode shown then you may be looking at a blank system database (of which I'm afraid as this would also explain your original plugin error). Go back to CMC > Objects & Folders and check if your documents are still there. Or, go to users & groups and check if your users are still there.
Let us know how you get on.