I encountered the same problem. At first I thought perhaps it was because I had upgraded MX in place (rather than reinstall), but then I uninstalled both MCSDK and STM32CubeMX and reinstalled both (5.Y.3 + 6.4.0). When I click "Generate" (or "Update" on an existing project) the progress bar / spinner at the bottom animates but otherwise nothing visible happens. I have tried leaving it running for over 30 minutes to no avail. I reinstalled 6.3 (alternate directory) and then selected it in MC Workbench and now it works again.
In case it's worth mentioning, I also momentarily encountered a problem after switching back to 6.3.0. The first time I tried to generate the project/code it displayed a red message saying "Updater busy". But then I ran STM32CubeMX separately (and changed the auto-update check to 99 days since I do not want it to update to 6.4.0 for obvious reasons), closed it, tried again in STMCW, and it worked fine.