After reply to a ticket get error message

Hi!

Today we made an upgrade to 4.71.2

The error is:

Class SwiftMailer_CharacterReaderFactory_SimpleCharacterReaderFactory does not exist (-1) in ./__swift/thirdparty/SwiftMailer/classes/SwiftMailer/DependencyContainer.php:309

What could be the problem?

Here are the whole trace:

Router: http://example.com/staff/index.php?/Tickets/Ticket/ReplySubmit/56717/inbox/5/4/-1/0
Uncaught Exception: Class SwiftMailer_CharacterReaderFactory_SimpleCharacterReaderFactory does not exist (-1)...

After reply to a ticket get error message