This commonly happens to me during the first run of sector 1 of Armada of Annihilation, and nearly every time the final boss luminmechs Dralberouge and Dralberoudi fail to spawn on my client, leading to my allies finishing them off and starting the next run of sector 1.
The resulting loading screen is then a never-ending one and I have to force-close the game and re-open it, after which I can normally play the rest of the Armada UQ in all sectors without issue.
This also just happened to me when the Dralberoudi was supposed to spawn in a story mission, resulting in me attempting to abandon the mission, again resulting in a never-ending loading screen I had to force-close and reopen the game to escape.
I've run file validation/verification twice now with no resolution to this issue.
Running the Steam version of the game, using PSO2 Tweaker utility for updates and settings.