Hi Feneberg,
Thank you for using our Syncfusion products.
In order to resolve the reported Assembly Manager issue, please follow the below workaround steps.
1. Open the Command Prompt with admin privileges and navigate to the following location.
Location: {ProgramFilesFolder}\Syncfusion\Essential Studio\{version}\Assembly Manager
Example: C:\Program Files (x86)\Syncfusion\Essential Studio\12.4.0.24\Assembly Manager
2. Please run the “
AssemblyManagerConsole.exe” with the below arguments.
/version:{version} /compilemode:prebuilt /action:install /framework:451,45
Refer the below image:

3. Now, 4.5 and 4.5.1 assemblies will be installed in GAC location and as well as in below ‘Assemblies’ location.
Location: {Drive:}\{Program files folder)\Syncfusion\Essential Studio\{version}\Assemblies\{4.5,4.5.1}
Example: C:\Program Files (x86)\Syncfusion\Essential Studio\12.4.0.24\Assemblies\4.5\
Note: We have fixed this issue in our upcoming volume release (2015 volume 1).
Please let us know if you need any further assistance on this.
Regards,
Kathiresan S.