I don't know if SoapUI developers read these forum (or how frequently they read it).
I opened an issue on GitHub: https://github.com/SmartBear/soapui/issues/534
For me, there is nothing in the SoapUI log tab that indicates an attempt to switch workspaces. The error log tab is blank.
Having the same issue.
For anybody who wants to revert to old version, here is a link to previous releases of SoapUI:
There is workaroud . Select workspace in version 5.5 ( instaled sepatately ) Then when soapui 5.6 start, it rememer, what was last selected in version 5.5