I'm not sure what the actual issue is here, but since a few days activities recorded with my edge 1040 do not sync to third party apps (e.g. Strava, Runalyze, ...). Exporting the activities .fit file (or accessing them via usb from the edge) and manually uploading them to the third party apps is however no issue. So it looks like the .fit files are correct.
Synchronizing activities from my Forerunner 955 has no issues and these show up on the third party apps.
The only recent changes I made was to add a Varia RTL 515 as connected devices (furthermore Rally 200, HRM Pro and my 8050 Di2 are connected via ANT+) together with the mybiketraffic.com connect iq app (as well as to setup the sync to mybiketraffic.com).
Otherwise, I only know that the recent activities (where I had my Edge 1040 with me and not just run efforts with the FR955) were all quite long ones >200km - not sure whether there is some "third-party-sync-size-limit" for actvities - an the added mybiketraffic.com information from the rtl 515 make problems there (the activity file is about 5.3MB).
Does anyone else has the same problem and/or does know how to solve it?