Postgres and CopyManager, Database connection failed when starting copy at org.postgresql.core.PGStream.receiveChar

Question

I'm trying to run copy mananger and load CSV file

val pgConnection = conn.unwrap(classOf[PgConnection]) 
val copyManager = new CopyManager(pgConnection)
val fis = new FileInputStream(tempTsvFilePath.toFile)
val copyCommand = s""" COPY $myTable($listOfMyColumns) FROM STDIN DELIMITER E'\t' CSV """
val linesCopied = copyManager.copyIn(copyCommand, fis)

It works file locally with TestContainers

val timesacaleDb = DockerImageName
    .parse("timescale/timescaledb:2.3.1-pg13")
    .asCompatibleSubstituteFor("postgres")

which starts with command

postgres", "-c", "fsync=off", "-c", "max_connections=500"

max_connections is temp hack until I throttle tests running in parallel.

Data is imported, all good, I verify every single imported record.

But exactly same code with exactly same input fails when I try to load data to real Postgres provided by ops.

org.postgresql.util.PSQLException: Database connection failed when starting copy
    at org.postgresql.core.v3.QueryExecutorImpl.startCopy(QueryExecutorImpl.java:949)
    at org.postgresql.copy.CopyManager.copyIn(CopyManager.java:45)
    at org.postgresql.copy.CopyManager.copyIn(CopyManager.java:220)
    at org.postgresql.copy.CopyManager.copyIn(CopyManager.java:203)

Caused by: java.io.EOFException: null
    at org.postgresql.core.PGStream.receiveChar(PGStream.java:455)
    at org.postgresql.core.v3.QueryExecutorImpl.processCopyResults(QueryExecutorImpl.java:1227)
    at org.postgresql.core.v3.QueryExecutorImpl.startCopy(QueryExecutorImpl.java:946)

I have no clue what it could be, what is the right way to debug it?