I was playing around with different Android Odoo-apps and one of these apps marked more than 800 previously read messages as unread on my server... Now I would like to set all messages to "read". 

I found this solution, but I can't get it working: https://www.odoo.com/de_DE/forum/hilfe-1/question/how-do-i-mark-a-set-of-messages-in-my-inbox-as-read-73961

As suggested, I selected all unread messages and clicked export. I chose "import compatible export" and I selected just "notifications/read". After exporting I get a file like that:

"id","notification_ids/is_read"
"__export__.mail_message_8003","False"
"__export__.mail_message_7992","False"
"__export__.mail_message_7984","False"

I changed "False" to "True" and tried to import. Odoo detects the columns correctly, but I am getting this validation error:

null value in column "partner_id" violates not-null constraint DETAIL: Failing row contains (14847, t, f, null, 8003). in Zeile 2
null value in column "partner_id" violates not-null constraint DETAIL: Failing row contains (14848, t, f, null, 7992). in Zeile 3
null value in column "partner_id" violates not-null constraint DETAIL: Failing row contains (14849, t, f, null, 7984). in Zeile 4

Any help welcome!