Hi there everyone!
=> tl;dr:

No image in DJI Go app;
app says "ready to go", aircraft flies normally;
records and takes pictures normally, histogram works fine;
tried on multiple devices (Android and iOS);
problem started after firmware update;
  - last time used with no problems:  July 18th 2020;
  - problem first noticed: August 15th 2020;

=> Long version:

I have a Phantom 3 4k and I'm getting no image in the DJI Go App.

All else works fine; Gps signal ok, warms up, takes off, flies normally.

I've seen posts of people getting "No image signal" in the status bar in the app, but mine says "ready to go"; so apparently the software "thinks" it's transmitting.

It even takes pictures and records video normally and the histogram in the app promptly shows any luminance variation.

I tried on an iPad 4, an iPhone 5, a Moto Z Play and on a Moto Z3 Play, so I'd guess it's not a problem with the device.

Last time I used with no problems was in July 18th 2020. On August 15th 2020 (less than a month later) I turned it up to check it before going out to fly it, and the app notified me of a firmware update. Updated it through the DJI Go app as with previous updates. When I started it again My cellphone wouldn't get any image even though all else worked fine and the system apparently detected no transmission error.

Talked to DJI online support, they said I should send the drone to the closest authorized tech support, which in my case is 400km (250mi). I wanna do that, but the entire proccess will take a long time, so i decided to try my luck here on the forums.

Anyone has any idea about what the issue can be and how to solve it?

Thanks everyone in advance.

Welcome Indiana Djanho i would try format the Sd card

Hi @JetSam.

I did. Tried 3 different SD cards, formatted them using the DJI Go app, Windows and OS X and the problem persisted.

Hello there Indiana Djanho. I am sorry for the trouble this has caused and thank you or reaching out. Kindly please follow the steps below for troubleshooting and to see if the issue will still persist?
1. Please remove your Micro SD card to see if the issue persists.
2. Were you still able to control the gimbal?
3. Please reset the Camera Settings to see if it helps.
4. Then please double check if the cable in the gimbal is loose, you may try to re-insert it.
5. If there is still no luck, it is recommended to send in the drone for the evaluation.
Thank you.