I constantly hit this issue in a scene graph based project. In a simple SG project it won’t happen when pushing only verse changes, but in a bigger project it happens way too easily.
One potential idea on how it may be reproduced (not guaranteed). Let the fortnite client go to sleep mode, compile new verse changes and trying pushing only verse. If the push fails, a subsequent full push might fail as well. Killing the valkyrie beacon will not help as you will bump into the upload error on launch. The only solution to this problem is to restart UEFN, which is annoying as it eats so much time.