Seems as though the problem is related to having NEW categories with sub categories in the csv file. (with colon separation)
e.g. "New Category:Subcategory"
It seems to me that MD has previously been able to handle this format on import and automatically add the category(s) and subcategory(s)
OK, it's a bug(ette)..... When you specify an account such as 'aaa:bbb' and when aaa does not exist, then it fails.... So when aaa exists, then it can create 'bbb'.... So perhaps create the top level accounts first.... Remove all second-levels, run the import, then delete all the txns, put the second-levels back, re-run the import.,,,