1. Go into your SpaceChem installation folder
2. Find SpaceChem.exe.config and open it with your favorite text editor
3. Delete everything
4. Copy and paste this code:
<?xml version="1.0"?>
<configuration>
<startup>
<supportedRuntime version="v2.0.50727"/>
</startup>
<runtime>
<NetFx40_LegacySecurityPolicy enabled="true"/>
</runtime>
</configuration>
5. Save the file
6. Enjoy