I have seen this bug in both GA and now in TW. It applies to the "Last Capital ship" as far as I can tell, and I've only seen it since the latest Title Update.
After placing 5 of my 6 total capital ships for TW defense, the 6th cannot be placed because the app hangs at the loading screen before I can choose ships for the fleet.
In GAC, this bug shows up after I've made 5 of 6 3 of 4 possible attacks, I cannot make the final attack because of the same hang at the same screen (spinning load screen before choosing a fleet.
EDIT JUNE 20: I HAVEN'T REPLICATED THE WORKAROUND BELOW IN SUBSEQUENT SIMILAR GAC SCENARIOS.
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I did find a workaround, by loading my account onto a secondary device (in this case, I experienced the bug on my Pixel 4 Android phone, and was able to use a 6th fleet to attempt to attack in GAC by accessing my account using the Nox Android emulator on Windows). I haven't verified that workaround for TW defense (yet).
EDIT: The workaround worked for the last attack in my GAC scenario, but is not working for the TW scenario.
I figured the workaround is good for people to know in the meantime, this is a frustrating bug.
swgoh account:
Jaesen 979-741-195