There is an issue with the migration process from 4.2 to 4.3.1 if you have relative date smart lists. On the second run of 4.3.1, BaseCamp will prompt that your list data is corrupted and it will revert to your backup (4.2.4) data. The only operational problem (other than the annoying prompt) is that any new list data is lost.
To work around this issue, delete and recreate any "date" smart lists using BaseCamp 4.3.1. There are many new options to date smart lists options that you can take advantage of. Any changes you make from that point on will be saved and the prompt will go away.
We are working on releasing 4.3.2 with the issue fixed as fast as we can, but the process typically takes a week. Making the software change is the easy part, but in the meantime the workaround should allow you to use 4.3.1 going forward without issues.