I tried that just now, and that indeed seems to have fixed it. Very strange!!

I'm now using my phone exactly the way I used my iPhone: one main account, which in the background imports the mail from the other gmail accounts and puts that mail in subfolders. This works for me of course, 'cause it has worked for 5 years like that before on the iPhone. But I do think it's a pity I cannot enter the other accounts directly, since that would have allowed me push mail on the other accounts as well. No biggie, but this seems like a bug to me.