Failing GSSAPI TCP when connecting to server

My application is trying to connect the database server, and meanwhile
tries to talk to the KDC server for a service ticket.
Earlier these TCP connections did run like this, and were successful:

13:57:53.788797 IP6 clientIPv6.54143 > serverIPv6.88: Flags [S], seq 4189109662, win 65535, options [mss 1440,nop,wscale 6,sackOK,TS val 402503231 ecr 0], length 0
13:57:53.789053 IP6 serverIPv6.88 > clientIPv6.54143: Flags [S.], seq 1604590824, ack 4189109663, win 65535, options [mss 1440,nop,wscale 6,sackOK,TS val 1505702993 ecr 402503231], length 0
13:57:53.789122 IP6 clientIPv6.54143 > serverIPv6.88: Flags [.], ack 1, win 1035, options [nop,nop,TS val 402503231 ecr 1505702993], length 0
13:57:53.789223 IP6 clientIPv6.54143 > serverIPv6.88: Flags [P.], seq 1:5, ack 1, win 1035, options [nop,nop,TS val 402503231 ecr 1505702993], length 4
13:57:53.829575 IP6 serverIPv6.88 > clientIPv6.54143: Flags [.], ack 5, win 1035, options [nop,nop,TS val 1505703037 ecr 402503231], length 0
13:57:53.829696 IP6 clientIPv6.54143 > serverIPv6.88: Flags [P.], seq 5:1166, ack 1, win 1035, options [nop,nop,TS val 402503271 ecr 1505703037], length 1161
13:57:53.832411 IP6 serverIPv6.88 > clientIPv6.54143: Flags [P.], seq 1:5, ack 1166, win 1035, options [nop,nop,TS val 1505703037 ecr 402503271], length 4
13:57:53.832440 IP6 serverIPv6.88 > clientIPv6.54143: Flags [FP.], seq 5:907, ack 1166, win 1035, options [nop,nop,TS val 1505703037 ecr 402503271], length 902
13:57:53.832461 IP6 clientIPv6.54143 > serverIPv6.88: Flags [.], ack 908, win 1021, options [nop,nop,TS val 402503276 ecr 1505703037], length 0
13:57:53.832620 IP6 clientIPv6.54143 > serverIPv6.88: Flags [F.], seq 1166, ack 908, win 1035, options [nop,nop,TS val 402503276 ecr 1505703037], length 0
13:57:53.832847 IP6 serverIPv6.88 > clientIPv6.54143: Flags [.], ack 1167, win 1035, options [nop,nop,TS val 1505703037 ecr 402503276], length 0

After an upgrade of the application they now look like this:

16:53:09.637708 IP6 clientIPv6.49124 > serverIPv6.88: Flags [S], seq 2860816940, win 65535, options [mss 1440,nop,wscale 6,sackOK,TS val 2664510889 ecr 0], length 0
16:53:09.637990 IP6 serverIPv6.88 > clientIPv6.49124: Flags [S.], seq 2595272492, ack 2860816941, win 65535, options [mss 1440,nop,wscale 6,sackOK,TS val 3896716896 ecr 2664510889], length 0
16:53:09.638153 IP6 clientIPv6.49124 > serverIPv6.88: Flags [.], ack 1, win 1035, options [nop,nop,TS val 2664510889 ecr 3896716896], length 0
16:53:09.638158 IP6 clientIPv6.49124 > serverIPv6.88: Flags [P.], seq 1:5, ack 1, win 1035, options [nop,nop,TS val 2664510889 ecr 3896716896], length 4
16:53:09.638162 IP6 clientIPv6.49124 > serverIPv6.88: Flags [FP.], seq 5:1166, ack 1, win 1035, options [nop,nop,TS val 2664510889 ecr 3896716896], length 1161
16:53:09.638451 IP6 serverIPv6.88 > clientIPv6.49124: Flags [.], ack 1167, win 1017, options [nop,nop,TS val 3896716896 ecr 2664510889], length 0
16:53:09.638508 IP clientIPv4.58934 > serverIPv4.88: Flags [S], seq 3474302110, win 65535, options [mss 1460,nop,wscale 6,sackOK,TS val 726603825 ecr 0], length 0
16:53:09.638696 IP serverIPv4.88 > clientIPv4.58934: Flags [S.], seq 2424341350, ack 3474302111, win 65535, options [mss 1460,nop,wscale 6,sackOK,TS val 2662337458 ecr 726603825], length 0
16:53:09.638729 IP clientIPv4.58934 > serverIPv4.88: Flags [.], ack 1, win 1027, options [nop,nop,TS val 726603825 ecr 2662337458], length 0
16:53:09.638733 IP clientIPv4.58934 > serverIPv4.88: Flags [P.], seq 1:5, ack 1, win 1027, options [nop,nop,TS val 726603825 ecr 2662337458], length 4
16:53:09.638751 IP clientIPv4.58934 > serverIPv4.88: Flags [FP.], seq 5:1166, ack 1, win 1027, options [nop,nop,TS val 726603825 ecr 2662337458], length 1161
16:53:09.638856 IP6 clientIPv6.46516 > serverIPv6.88: Flags [S], seq 3686126664, win 65535, options [mss 1440,nop,wscale 6,sackOK,TS val 1901751531 ecr 0], length 0
16:53:09.639054 IP serverIPv4.88 > clientIPv4.58934: Flags [.], ack 1167, win 1009, options [nop,nop,TS val 2662337458 ecr 726603825], length 0
16:53:09.639071 IP6 serverIPv6.88 > clientIPv6.46516: Flags [S.], seq 3830196306, ack 3686126665, win 65535, options [mss 1440,nop,wscale 6,sackOK,TS val 457425497 ecr 1901751531], length 0
16:53:09.639105 IP6 clientIPv6.46516 > serverIPv6.88: Flags [.], ack 1, win 1035, options [nop,nop,TS val 1901751531 ecr 457425497], length 0
16:53:09.639130 IP6 clientIPv6.46516 > serverIPv6.88: Flags [P.], seq 1:5, ack 1, win 1035, options [nop,nop,TS val 1901751531 ecr 457425497], length 4
16:53:09.639154 IP6 clientIPv6.46516 > serverIPv6.88: Flags [FP.], seq 5:1166, ack 1, win 1035, options [nop,nop,TS val 1901751531 ecr 457425497], length 1161

This is repeated LOTS of times, and what remains is hundreds of
sockets in FIN_WAIT_2 that do not appear to disappear quickly.
The error message from postgres client is
GSSAPI continuation error: Miscellaneous failure: unable to reach any KDC in realm
(In fact in does reach the KDC, only the TCP doesn't seem to work)

A configuration problem on the machine(s) can be ruled out, because both
old (working) and new (failing) application are installed on the same
machine at the same time, using the same network, same hardware, same
OS, same libgssapi and same postgres client software connecting to the
same database.

There are no errors logged on the KDC server, it appears to have
orderly processed the requests (at least at first, it starts to
complain later when the stale sockets get too many).

The error message on the postgres server is
FATAL: GSSAPI authentication failed for user "pmc"

The OS is FreeBSD Release 13.4-p1
The postgres client library libpq.so.5 is Release 15.7
The application postgres interface is rubyGem pq Release 1.5.4
The application is Discourse 2.2.0 (working) and 2.3.1 (failing)

What is going on there?