Critical errors during logical decoding

List,

For seemingly no reason at all, my logical replication slot has just
started bailing out with errors every time my client tries to connect to
it. Particularly - at this moment - I'm getting:

2018-02-07 19:14:31 UTC [3985-1] svc_app(at)app LOG: 00000: received
replication command: START_REPLICATION SLOT event_stream LOGICAL 0/0
("startup_params_format" '1', "no_txinfo" 'true', "expected_encoding"
'UTF8', "max_proto_version" '1', "proto_format" 'json', "min_proto_version"
'1')
2018-02-07 19:14:31 UTC [3985-2] svc_app(at)app LOCATION:
exec_replication_command, walsender.c:1341
2018-02-07 19:14:31 UTC [3985-3] svc_app(at)app LOG: 00000: starting logical
decoding for slot "event_stream"
2018-02-07 19:14:31 UTC [3985-4] svc_app(at)app DETAIL: streaming
transactions committing after 0/8DDFD48, reading WAL from 0/8DDDC00
2018-02-07 19:14:31 UTC [3985-5] svc_app(at)app LOCATION:
CreateDecodingContext, logical.c:408
2018-02-07 19:14:31 UTC [3985-6] svc_app(at)app LOG: 00000: logical decoding
found consistent point at 0/8DDDC00
2018-02-07 19:14:31 UTC [3985-7] svc_app(at)app DETAIL: There are no running
transactions.
2018-02-07 19:14:31 UTC [3985-8] svc_app(at)app LOCATION:
SnapBuildFindSnapshot, snapbuild.c:1245
2018-02-07 19:14:31 UTC [3985-9] svc_app(at)app ERROR: XX000: no known
snapshots

Other errors have included missing toast entries, as well as missing base/
files on disk. I blew away the entire data directory, and restored the
database from a pg_dump to a fresh directory after checking filesystem
consistency. The logical replication stream operates for a few minutes, and
then eventually bails out again. This is Postgres 9.6.6 on Ubuntu installed
from packages.

Does anyone have any insight into what could be happening here? Other steps
to try and rectify the problem? The database itself does not appear to have
any issues that I can see.

Best,
Colin