From your error, server and client should use same version.
Need to upgrade license server?
It doesn't matter what version of FlexLM you have because SolidWorks incorporates it into the SolidNetwork License (SNL) manager. In order to run SolidWorks 2019 you need to:
- Deactivate your licenses on the server.
- Install the SW2019 SNL
- Then, reactivate the licenses.
Make sure you can access the www.solidworks.com website on the server before you deactivate the licenses, otherwise you will be out of licenses until you get the server connected to the Internet.