libpq and mysterious "invalid byte sequence for encoding UTF8".

I'm stuck trying to find a cause for

> invalid byte sequence for encoding "UTF8".

It is an C program using libpq. I'm using `PQexecParams` to
execute the SQL query.

The offending byte sequence is completely random, sometimes the command
even runs ok. I thought I must have a memory allocation issue somewhere,
but even if I specify all the parameters as static strings, I still
receive the error with a random byte sequence.
What's more, the same query with the same parameters runs ok when I
create a small test program.
So I'm completely stuck. I verified all the possible sources for the
error like client_encoding etc, but could not find the source of the error.
What is confusing me is that the offending byte sequence is random, even
though the query parameters don't change.
Moreover, when I check the postgres log, the query and its parameters
appear to be correct.

I'm trying to update a record in the following table:

    CREATE TABLE public.contacts
    (
        contactid integer NOT NULL DEFAULT
nextval('contacts_contactid_seq'::regclass),
        paperid integer,
        pos character varying(50) COLLATE pg_catalog."default",
        title character varying(10) COLLATE pg_catalog."default",
        firstname character varying(20) COLLATE pg_catalog."default",
        lastname character varying(25) COLLATE pg_catalog."default",
        func character varying(25) COLLATE pg_catalog."default",
        tel1 text COLLATE pg_catalog."default",
        tel2 text COLLATE pg_catalog."default",
        fax1 text COLLATE pg_catalog."default",
        fax2 text COLLATE pg_catalog."default",
        email1 character varying(50) COLLATE pg_catalog."default",
        email2 character varying(50) COLLATE pg_catalog."default",
        maincontact boolean DEFAULT false,
        publdatacontact boolean DEFAULT false,
        invcontact boolean DEFAULT false,
        queries_recipient boolean,
        contact_log text COLLATE pg_catalog."default",
        salesforceid character(18) COLLATE pg_catalog."default",
        fakelastname boolean NOT NULL DEFAULT false,
        CONSTRAINT contacts_pk PRIMARY KEY (contactid),
        CONSTRAINT contacts_paperid_fkey FOREIGN KEY (paperid)
            REFERENCES public.papers (paperid) MATCH SIMPLE
            ON UPDATE CASCADE
            ON DELETE CASCADE
    );

Here is an actual code:

        const char* pparams[16] = {
    NULL,
    NULL,
    "1702",
    NULL,
    NULL,
    NULL,
    NULL,
    NULL,
    NULL,
    NULL,
    NULL,
    NULL,
    NULL,
    NULL,
    NULL,
    "14340"
        };
   

   
    gchar *query="UPDATE contacts SET

Pos=$1::varchar,Title=$2::varchar,PaperID=$3::int,FirstName=$4::varchar,LastName=$5::varchar,Func=$6::varchar,Tel1=$7::text,Fax1=$8::text,Email1=$9::varchar,Tel2=$10::text,Fax2=$11::text,Email2=$12::varchar,MainContact=$13::boolean,PublDataContact=$14::boolean,InvContact=$15::boolean
WHERE ContactID=$16::int";
   
          result = PQexecParams(conn, query, 16, NULL, pparams, ssizes,
bbinary, 0);

An excerpt from Postgres log:

    Jan 26 08:40:57 ip-172-16-10-94 postgres[11334]: [113-1] 2021-01-26
09:40:57.505 CET [11334] jira@project-syndicate LOG:  execute <unnamed>:
   
        UPDATE contacts SET Pos = $1::varchar, Title = $2::varchar,
PaperID = $3::int, FirstName = $4::varchar, LastName = $5::varchar, Func
= $6::varchar, Tel1 = $7::text, Fax1 = $8::text, Email1 = $9::varchar,
Tel2 = $10::text, Fax2 = $11::text, Email2 = $12::varchar, MainContact =
$13::boolean, PublDataContact = $14::boolean, InvContact = $15::boolean
WHERE ContactID = $16::int
        Jan 26 08:40:57 ip-172-16-10-94 postgres[11334]: [113-2]
2021-01-26 09:40:57.505 CET [11334] jira@project-syndicate DETAIL: 
parameters: $1 = NULL, $2 = NULL, $3 = '1702', $4 = NULL, $5 = NULL, $6
= NULL, $7 = NULL, $8 = NULL, $9 = NULL, $10 = NULL, $11 = NULL, $12 =
NULL, $13 = NULL, $14 =  NULL, $15 = NULL, $16 = '14340'
        Jan 26 08:40:57 ip-172-16-10-94 postgres[11334]: [114-1]
2021-01-26 09:40:57.544 CET [11334] jira@project-syndicate ERROR: 
invalid byte sequence for encoding "UTF8": 0x80

Any ideas as to what could be a cause of the error?