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Where Elastic is telling you that? Could you tell?
About the logs, it seems to indicate that at least one of you document in the index is not really readable (not a correct JSON)... Not related to a disk space issue or whatever else.
Could you run:
GET /idx_siv/_search?size=10000
To see if you can reproduce that?
What is the output of:
GET /_cat/indices/idx_siv?v