That might be one for Ronski to look into as he coded the GUI.
However, from a quick google, it suggests that a file couldn't be found.
I think the GUI tries to read data from modem_stats.log and xlogfile.txt, both of which should be located in the Modem_Log folder specified in the HG612_stats.ini file & often the default locations as shown in the attachment.
These are the default locations in the ini file as downloaded :-
[Locations]
Main_folder= C:\HG612_Modem_Stats
Scripts_and_Programs= C:\HG612_Modem_Stats\Scripts
Apps_folder= C:\HG612_Modem_Stats\Apps
Current_Stats_folder= C:\HG612_Modem_Stats\Current_Stats
Ongoing_Stats_folder= C:\HG612_Modem_Stats\Ongoing_Stats
Modem_Log= C:\HG612_Modem_Stats\Ongoing_Stats
If the locations as shown in the GUI, assuming you can get that far, are shown in yellow, it would suggest that they are the 'wrong' locations and/or that those folders don't actually exist.
The default locations can be changed via the 'Browse' button in the GUI or by manually editing the ini file, but they do need to be folders that actually exist.