Some users have problems opening the IGC files in GPSDump when there is more than one day of tracklog. You can open the IGC file directly in a text editor like Notepad, and remove the lines of log from the wrong day. Save as IGC file again, not .txt.
It should then be possible to open and convert to DL format in GPSDump. You need a recent version of GPSDump, it can be downloaded from Stein Sørensen’s homepage