I successfully uploaded a csv formatted file, so the channel still works.
But now I realise I messed up! I deleted channels and created new ones and I think I used the Read API and not the Write API to upload data.
Not at home right now so I cannot test but I think the problem is solved.
I do use 10 times esp8266, I will be giving a workshop on IoT and each participant will use its own esp8266.
For testing purposes it's a cluster attacthed to 10 port USBhub.