Thanks for your reply!

The new import worked for me, although I have only imported one address

Just to let you know, when I go to the "Fork Balance Window" and click on "Update Txs" I get a crash: System.OverflowException: 'Value was either too large or too small for an Int32.'

(This seems to happen regardless of whether a specific fork has been chosen from the drop-down or a custom block height has been entered.)