The error log shows:
StructureMap.Building.StructureMapBuildException: Failure while building 'Lambda: new Database(value(LMC.Toolbox.Server.StartUp.StructureMapBootstrap+<>c__DisplayClass35).connectionStringName)'.
 

Fix:
  1. Go to Toolbox server's Bin folder (default folder C:\Program Files (x86)\QSAR Toolbox\QSAR Toolbox 4.2\Toolbox Server\Bin).
  2. Delete the ToolboxServerWindowsSvc.exe.config file
  3. Rename a copy of ToolboxServerApp.exe.config to ToolboxServerWindowsSvc.exe.config. Make sure you don't just rename the original file.
  4. Restart the service.