Found out today that there was an update, 4.3.4, for Basecamp for OS X. The text said "Fixed issue where list data was misread during upgrade from 4.2". THIS IS NOT TRUE! I still have exactly the same error. Alla data is shown in ONE folder called "Unlisted data". I have added two screenshots to show what I mean. The data itself does not seem to be corrupt, just put together in one single folder; Installing 4.2.4 again makes the lists show correct data again. Sigh, I guess I have to wait again for a new version ...
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Furthermore, my cloud data seem to have gone in the new 4.3.4.