Steam - failed to load steamui.dll


I have been using Sandboxie for Steam for some months, so I have an extra account on my laptop. 

The last few days I have encountered some trouble with downloading games with my Sandbox Steam, so I decided to remove the sandbox and create a new one.

When I tried to open launch Steam after I created a new sandbox it game me this message;   Anyone encountered this before? I searched a lot of Steam forums, and couldn't find a solution, which fixed my problem.

I would like to add that I recently have updated to the newest Windows 10 update.

Thanks in advance!