Last 2 times I've had this, I got exactly the same thing. My first action is to report it to support. Not sure if that does much good but who knows. In the meantime I delete app data, uninstall app, restart phone, reinstall app. Wait for game data to download and log back in once the start button appears.
I can then get back into the game which runs *really* slow. Clear any spam about buying souls then go straight to 'my team' and 'sell'. It will take a long time but I try and sell off a few evo runes and shards - anything I had multiple stacks of, so that I have fewer stacks of stuff overall. If I have the patience I might clear out spare talismans and any 1* or 2* junk toons.
Once that is done the game should respond a little quicker but may need a restart. Last time I had to repeat the process twice bit game has been fine since. I also try to avoid building up any large surplus materials.