The sandbox configuration is stored in the Sandboxie.ini, which is typically located in C:\Windows, while the user interface of Sandboxie-Plus stores its settings in the Sandboxie-Plus.ini, which is typically located in the user folder C:\Users\[username]\AppData\Local\Xanasoft\Sandboxie-Plus.
Copy the files to some safe place and move them back.
For the contents of the sandboxes, you have to back up the folders along with all the files. The default path is C:\Sandbox\
1) Copy Sandboxie.ini, Sandboxie-Plus.ini and the folders of the sandboxes to a safe place.
2) Install Sandboxie-Plus again. There is no need to run it.
3) Copy Sandboxie.ini, Sandboxie-Plus.ini and the folders of the sandboxes back to their original places.
Sandboxie has a feature to export and import boxes (right-click on a box, select Sandbox Tools > Export Box; Sandbox > Import Box), which covers the files of a sandbox and its settings, but that can be more time-consuming, depending on the amount of boxes.