12.1 not useable: clientlib fails after a dozen queries (GSSAPI ?)

Hi folks,

with 12.1, after a couple of queries, at a random place, the clientlib
does produce a failed query without giving reason or error-message [1].
Then when retrying, the clientlib switches off signal handling and
sits inactive in memory (needs kill -9).

The server log shows no error or other hint.
The behaviour happens rarely with trust access, and almost always when
using Kerberos5 (Heimdal as included in FreeBSD).

11.5 clientlib has none of this behaviour and seems to work fine, like
10.10 did.

Environment:
    OS      FreeBSD 11.3
    Applic.    Ruby-on-Rails, ruby=2.5.7, gem 'pg'=1.2.2
            (it makes no difference if that one is compiled with
        the 12.1 or the 10.10 library)
    Server  12.1

[1] the message from ruby is
    PG::ConnectionBad: PQconsumeInput() : <query>

rgds,
PMc