I can confirm the SPSS File download issue of the OP by using https and Internet Explorer 7 and 8.
We used Limesurvey 1.72 (5737) for a long time, and we always had that problem. We updated our installation about 5 minutes ago to the most recent Version 1.87+ Build 8518, still the same issue.
There is no problem downloading the SPSS file using alternate Browsers like Firefox, only Internet Explorer 7/8 have this issue.
(offtopic edit: strange, i am still called a "fresh lemon"? we donated a while ago, and we got a promise that our forum status has been changed because of our donation... well, not that I care very much about it
edit2: never mind, i just had to take a look into my profile... doh )