Been getting this error lately trying to play a game in Sandboxie-plus in Win 10. Seems to happen after trying to Load/Update the 2nd steam. Was working fine until a few weeks ago or so.
If it worked before, Sandboxie should not be causing this, unless ProtectHostImages=y is being set and the .dll got updated in the sandbox, while the application trying to load it is located on the host machine (outside the sandbox).
Sandbox Options > Various Options > [Dlls & Extensions] > Image Protection > "Prevent sandboxed programs installed on the host from loading DLLs from the sandbox"