Thanks for the suggestion but on closer examination it looks like even if this method were to work temporarily it would definitely not be a long term solution for me because there is no way that I am going to buy a $59/year mobi subscription just to get back the functionality I have had for years and years. I will try rolling back to an older version instead.
It is a pity that Garmin does not offer the option to download your activities as a *.fit file from Garmin Connect. You can only export as CSV but Sporttracks can not read that file when I try to import.
Well, I guess Garmin made this change because they no longer want people to use anything other than their Connect service. You know, BIG DATA and so on...
Nice try, Garmin. I'll make sure that my next fitness watch is not one made by Garmin then.
The 910XT has been getting on my nerves recently anyway. It always takes ages until I get a GPS signal and standing still in the cold waiting for satellites to be found is not exactly fun. I have a sneaking suspicion that Garmin might have also nerfed GPS connectivity of the 910XT via an update to get people to buy new devices.
#notamused

... oh well, I guess I will just add workouts manually to Sporttracks for now. Thanks for pointing out the mobi option though. It's interesting to know that exporting *.fit from Connect is technically possible, i.e. Garmin apparently just chooses not to offer that option for reasons...
