Special characters get messed up when importing RSS feeds

Dear wordpress-friends.
After I migrated our site to another server using Duplicator, I suddenly experience problems fetching RSS feeds using both WP RSS Aggregator AND FeedWordpress, as special characters (??ü??ü?’ and others) get messed up, resulting in “T??r” instead of “Tür”.
I tried adding an unicode converter for Aggregator, but even this didn’t work.
I expect this to be a database problem, as BOTH tools are having problems. They didn’t have these issues on the old server.
However I can easily setup a post with content ??üü???`′ and it gets displayed correctly.
My DB connection is set up as utf8mb4_unicode_ci, the tables use the same collation. WordPress is set to
define(‘DB_CHARSET’, ‘utf8mb4’);
Any idea what might be wrong here?
Thanks a lot!