@Camtajoie Workaround: It loads the last 1000 messages, then deletes the 300 oldest from the list. By reloading Smart Messaging, it will thus have access to messages 1001-1300 which will thus be part of the 1000.
It can repeat the operation until all unread messages are deleted 
I have to delete messages from the messaging system via smart messaging, is that it?
Until maybe finding the one that is causing the problem?
Yes, exactly! I was going to suggest the same solution as @WinRar: delete some messages and after a while you will reach this notification/message.
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Thank you very much! Good idea!