I have the same problem. But not only when logging in, also in dungeons when I need to leave the victory screen, loading take very long or I am locked in the battlescreen and need to refresh again with the issue you describe =(.
It’s very annoying and I have this loadingproblem and the lag that comes with it like 2 weeks now.
And yes cleaned cookies, browser and so on, even connected my computer straight to the modem of my internet to see it was a wifi problem on my side, but nope it isn’t either. I really think it’s something in the game or the servers? Really no clue…