Hi Lenny,
Sorry for the inconvenience caused.
We have checked the reported install
error in the same scenario. We are not able to reproduce the reported install
error from our end. Please follow the steps mentioned below to resolve the
uninstall issue.
1.Download the Syncfusion Install
Cleanup.zip file from the below link.
https://s3.amazonaws.com/files2.syncfusion.com/Installs/Support/Syncfusion%20Install%20Cleanup/Syncfusion%20Install%20Cleanup.zip
2.Extract the downloaded zip file.
3.Please run the
SyncfusionCleanupWindow.exe
4.Pop up window will be open as like
the below screenshot. Select the Syncfusion Essential Studio version which you
want to uninstall and then click Remove.
Please refer the screenshot below.

5.Now, you can reinstall the
corresponding version Essential studio setup with the respective version unlock
key.
Still if you face the same issue, we
suspect that setup has been corrupted. So can you please re-download the
Syncfusion Essential Studio setup v11.1.0.21 and install the downloaded setup
with the respective unlock key?
Please let me know, if you have any
concerns.
Regards,
Vadivel N