TLS in KMail not working after Buster upgrade

Dear all, I just did an upgrade from Debian Stretch to Buster. All seems to have went well, except one thing. When KMail attempts to fetch mail via IMAP from a server that requires TLS I get an error saying "Error initializing TLS". (Or something similar - I'm translating from Polish. The exact message in Polish is "Błąd podczas uruchamiania TLS".) Any ideas? Janek --- Politechnika Łódzka Lodz University of Technology Treść tej wiadomości zawiera informacje przeznaczone tylko dla adresata. Jeżeli nie jesteście Państwo jej adresatem, bądź otrzymaliście ją przez pomyłkę prosimy o powiadomienie o tym nadawcy oraz trwałe jej usunięcie. This email contains information intended solely for the use of the individual to whom it is addressed. If you are not the intended recipient or if you have received this message in error, please notify the sender and delete it from your system.