Thank you for contacting Syncfusion Support.
We suspect that the GAC assemblies were not properly configured or previous version of Syncfusion assemblies were not removed properly from your machine. Please refer to the below link to ensure whether the GAC assemblies are installed properly in your machine.
And also ensure whether the Syncfusion controls are configured properly for the .NET Framework version using the below steps:
· Close your application and clear bin/obj files.
· Open Essential Studio and then select Utilities.
Select Assembly Management and then select Assembly Manager Launch option.
Remove all the installed versions from your machine.
Select the Installed version and framework.
Select Perform Action to start the installation of controls.
Please close the application and clean the bin and obj folder of your application and rebuild the project again and make sure that the Syncfusion assemblies are properly referred to and try to use ButtonAdv in Form Designer.
If the above solution doesn’t help, please close the application and clean the bin and obj folder of your application and rebuild the project again. Delete the mentioned control from your Toolbox and add it again using the following steps.
Delete the Button from Toolbox as mentioned in the screenshot.
Add it again as mentioned in the screenshot.
Add ButtonAdv based on the Framework you haved used in your Visual Studio as mentioned in the screenshot.
The ButtonAdv will be added in ToolBox and it should be working properly now.
Please do the above and let us know if it helped.