ERROR: no known snapshots

Hello,

We have few highly available clusters with patroni managing PostgreSQL
9.6 (pgdg on debian stretch). We use logical decoding with wal2json to
stream data to a remote platform. On a majority of clusters, it works
like a charm. But for one, we have the following error right after
decoding start:

[1353-1] LOG: starting logical decoding for slot "*****"
[1353-2] DETAIL: streaming transactions committing after 321/292C06F0, reading WAL from 321/28BA9898
[1354-1] LOG: logical decoding found consistent point at 321/28BA9898
[1354-2] DETAIL: There are no running transactions.
[1355-1] ERROR: no known snapshots
[1355-2] CONTEXT: slot "*****", output plugin "wal2json", in the change callback, associated LSN 321/2A985A30

The only reference of the "no known snapshots" error is in PostgreSQL source
code:
https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/access/heap/tuptoaster.c;h=ade4bd142dfbd7ef3340495b06d1041e4032a17f;hb=ca3489e768990e32096f0f24d46d5f0b8b70cff4#l2320

Do you know what can cause this error?

Julien