A page Preview is unavailable error is generated when attempting to view the preview while hovering over a link of a page, if the page is a redirect.
I cannot reproduce with a clean redirect link. (in Firefox 65.0.1 (Linux) nor in Chromium 71)
You can Test it at https://en.wikipedia.org/w/index.php?title=User:Quiddity_(WMF)/sandbox&oldid=885582095
My result is a proper HTTP 302, and fully functional PagePreview:
(Links and examples always help! Please try to make life easy for each other :-)