Aehm, what exactly did you do to get the error message? I could import the flight plan at first try and I then could also export the flight plan to the CSV as intended.
To create your Route Map you have to go to the Create page and select the world and 2561 as your airline ID. Once the flight plan is imported, click on the "View this airline" link at the bottom. Obviously, as I have already imported it now, you can also directly select it using the View page.
Then click the Menu button and choose "Export Flight Plan".
for some reason, Firefox is not formatting this properly
After I saved the file in Chrome and IE, it formatted this differently, and opened fine in Excel
I tried a few things but it seems that Firefox is adding some line breaks for easy and convenient reading of text pages/files. There is an option under "View - Page Style - No Style" and also an option under about:config. Yet still having tried both of them, the saved text file still has the line breaks.
One possibility to use Firefox is to view the source and then save the file. This way it worked for me.
Or as you wrote, good old IE is not messing up the output.
On another note, I have added an option to choose the type of the times. By default it will output UTC time.
If you manually add &T=L (or &T=H) to the URL then you can display the Local times or the Hub times for the departure/arrival times.
I have also added the Distance, Duration of the flight and the average speed (in km/h).