Yes, and it is very simple. Rename the file to the .zip extension. However, that is not the problem. The problem is backwards compatibility. There should be no reason why it has stopped, unless there has been a significant database change from v2.3.1+ to 2.3.2. I don't think that is the problem, so it has to be something else. Is there a plugin or block in the v2.3.2 that you are not using in the 2.3.1+? Has there been a mod in either that is not present in the other? These are the most likely candidates for preventing a restore from working. When startign the restore, did you get any kind of error/warning/notice message? Think about these things as well as, when you made the backups, was there any issue that arose? Was there any kind of error/warning message there?
Oh.. another thought. When I do backups of courses for transfering to another Moodle, I never, never include student/user data, nada, nothing. Is it possible that user data has been included here? Think about it...