Detected in version 0.5 and also found in version 0.5.1
1) open menu File->Import->Import CSV File...
2) select transaction type 'Expenses' and press Next
3) enter a file name and press Next
4) select all 'Column' radio buttons to read in all data from file, including 'Category' (assuming that all categories indeed exist, which should be true if you import a previously exported and unmodified file)
5) untick 'Create missing categories and accounts' and press Finish
This is answered by a message like 'Unable to import any transactions imported. Failed to import N data rows. -Unknown category found.
There are two workarounds: either allow the application to create missing categories and accounts (ending up with doublicate categories) or select the 'Value' radio button for 'Category' and correct the categories manually afterwards.
tried to solve it with patch 2347204