The workspace preview does not work with lockSSL=3 being defined in the Install Tool. First it looks like a CSS issue, in the end it boils down that the preview IFRAMEs are not correctly initialized in \TYPO3\CMS\Workspaces\Hook\TypoScriptFrontendControllerHook. The mentioned hook is only called once for the request to the first IFRAME showing the website frontend preview and won't be called for further.
The reason is, that cache expire headers are sent that prevent the client to update the preview on each preview request. That's why the mentioned hook is not called and the IFRAMEs are not initialized properly. No-Cache headers are sent if a backend user object is initialized correctly - and that's the actual bug, a wrong but strict PHP condition, comparing a string (lockSSL) with an integer value.
The comparison flaw was integrated with issue #54265