Try to go to the Control Panel/Administrative Tools and check the Microsoft Dynamics AX 2012 Server Configuration and note if you have 'Enable the hot-swapping of assemblies for each development session' activated.
 
If the 'Enable the hot-swapping of assemblies for each development session' parameter is activated, try to deactivate it, press Apply and say Yes to restarting the AOS Service. Then try to go to the AOT/Tools/Options and see if you are not able to enter the Options screen without any error

To Enable Hot Swapping of Assemblies

  1. Open the Microsoft Dynamics AX server configuration utility by clicking Start > Administrative Tools > Microsoft Dynamics AX 2012 Server Configuration.
    To change the server configuration, you must run the utility with administrative privileges.
  2. On the Application Object Server tab, select Allow hot swapping of assemblies when the server is running.
  3. Click OK. You will see a prompt that asks whether you want to restart the AOS service. The change will only take effect after you restart the AOS.

To Turn Off Hot Swapping

After you have completed your code change and have compiled the assembly DLL for the last time, we recommend that you turn off hot swapping.
To turn off hot swapping, you clear the Allow hot swapping of assemblies when the server is running check box, and then restart the AOS. This ensures that all connections use the same version of the DLL.