This error can occur when opening SolidWorks even though the activation process reports that activation with the current serial number has been successful.

To resolve this error please try the following.
Open File Explorer and go to C:\ProgramData\FLEXnet.
Rename the FLEXnet folder to something like FLEXnet_old.
(The ProgramData folder is a hidden folder. You can either use 'Folder options' to show hidden folders, or you can type C:\ProgramData directly on to the address bar and then press Enter to access this folder. From there you will be able to locate the FLEXnet folder.)

Activate the license again. 
Then you should be able to open the software.

If you need further assistance please email webstore@illinois.edu