This error can mean many different things, but most likely you have run out of space in one of your lists or in your .CLF file containing your lists. The first thing to do if you receive "invalid block number" is to make a backup of your ListPro data on your desktop PC, if possible.
Make a backup by picking File -> Make Backup, and then give the backup file a unique name (such as the date.)
Then, try compacting your data file by right-clicking on the name of your list file, and then picking Compact File. If compacting doesn't work, you may be exceeding one of ListPro's size limits. Each list in ListPro is limited to 3MB of data, and the entire .CLF file is limited to 16MB. If you are adding an item to a list and receive this error, check the size of the file; if it's under 16MB, you are trying to add more than 3MB of data to a list. If the file size is at or near 16MB, you're exceeding the file size limit.