I had the exact same problem upgrading to 2.4.0. Never had any issues with solo upgrades in the past. This time solo would never connect to the app. Waiting on Solo and vehicle not found bullshit. I logged a ticket and after trying many reboots I just figured I'd do the factory reset which fixed the issue but I am noticing that on occasion when I first power up solo and controller I don't get video on the screen and then it's on to rebooting the GoPro, solo app, etc.
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I have noticed that if you power up the camera before anything else, things seem to go better with respect to the video feed.