[GENERAL] Invalid field size

I've implemented a backup procedure in C# with Npgsql (using COPY TO I
dump all tables in a compressed file) that's been working well in the
last 5 years (and it's still working, since this is a single, isolated
case).

OS: Windows 7
PG: 9.1.6 (I know, it's EOL, but I think it's not matter here)

While restoring (with COPY FROM) I get this error:
2017-07-04 12:55:27 CEST ERROR:  invalid field size
2017-07-04 12:55:27 CEST CONTEXT:  COPY tab, line 619, column thumbnail
2017-07-04 12:55:27 CEST STATEMENT:  COPY
tab(cod,guid,data,blob,thumbnail,descr,type,url,user,home,codrec,table,op,dagg,last)
FROM STDIN WITH BINARY

with this table definition:

CREATE TABLE public.tab
(
   cod uuid NOT NULL DEFAULT uuid_generate_v4(),
   guid uuid NOT NULL,
   data timestamp without time zone NOT NULL,
   blob bytea,
   thumbnail bytea,
   descr character varying(255) DEFAULT NULL::character varying,
   type character varying(50) DEFAULT NULL::character varying,
   url character varying(255) DEFAULT NULL::character varying,
   user character varying(255) DEFAULT NULL::character varying,
   home character varying(255) DEFAULT NULL::character varying,
   codrec uuid,
   table character varying(30) DEFAULT NULL::character varying,
   op character(1) DEFAULT NULL::bpchar,
   dagg timestamp without time zone,
   last character varying(16) DEFAULT NULL::character varying
)

As you can see I have 2 bytea fields, blob and thumbnail (the one it
seems it's giving the error), but AFAIK the former is never used, so it
should be always null.
Googling around did not help.

Any ideas? As for many error I got in the past I assume we are trying to
COPY FROM corrupted data (when using cheap pendrives we get often this
error). Should it be reasonable or I have to search elsewhere?

Thanks in advance
Moreno.