I'm running into the same issue, it seems the preview URL is adding a /- to the beginning, for some strange reason. Removing it generates a 500 error instead of 404 error (even log shows the url expired, so it's valid without the /- in there, but you cant just modify it for testing).
I'm checking with Kentico support on this one...
Gives a 404
/cmsctx/pv/tfayas/culture/en-US/wg/a08527a3-7607-4cde-83d0-3d09d5115535/readonly/0/ea/1/h/78a6c1070893a2757c85d9570cfd4678cf6bc17568dd6b68ca6c08481239a356/-/home?uh=5435fcbb42a22a062c029f29fa81d0aa65a6e6a0fdbda1c74fa06aeca346ce71&editmode=1&administrationDomain=http%3A%2F%2Fadmin.bibleanalytics.org
Gives a 500
/cmsctx/pv/tfayas/culture/en-US/wg/a08527a3-7607-4cde-83d0-3d09d5115535/readonly/0/ea/1/h/78a6c1070893a2757c85d9570cfd4678cf6bc17568dd6b68ca6c08481239a356/home?uh=5435fcbb42a22a062c029f29fa81d0aa65a6e6a0fdbda1c74fa06aeca346ce71&editmode=1&administrationDomain=http%3A%2F%2Fadmin.bibleanalytics.org