Same here, but only for the arcor account of my wife. I asked the support of vodafone to re-open POP3 / IMAP access again yesterday, but no reaction yet.
Mine is still working also in K9. I haven’t seen a difference in the settings of our two accounts so far, but I’m not so familiar with the settings at all, as we used K9 since years and it just worked.
So if you have ideas, where to compare the 2 accounts, please let me know.