Up until version 6.1, this app was rock solid. I have not been able to tag any files since updating to version 6.1. Either the problem lies with the new version of the iOS app, or with the new version (4.0) of the desktop app. In any event, its too bad because this used to work flawlessly.
***UPDATE*** As mentioned above, version 6.1 had some sort of bug in it. I reported this to the author and he got back in touch with me almost immediately. Using the error message I received and my log of recent trips, he immediately found the problem, created a new test version of the app, sent me the new app and a custom profile so I could install it on the phone and not have to wait for Apples review process. Everything now works as it should again and Apple accepted the new 6.1.1 update very quickly as well so gps4cam is rock solid again. Extremely fast responses from this developer!
Tristar about gps4cam - Geotag Your Photos, v6.1.1