can't enter passphrase in pinentry

running on
Intel(R) Core(TM) i5-3320M CPU @ 2.60GHz

Xubuntu
Ubuntu 16.04.5 LTS, xubuntu-core 2.206

Thunderbird Version 60.2.1
with Enigmail Version 2.0.8 (20180804-1515)

using
$ gpg2 --version
(GnuPG) 2.1.11

A pinentry window opens outside the terminal as separte window when commanding
$ pinentry <<EOT
SETDESC Hello World
CONFIRM
EOT

$ cat $HOME/.gnupg/gpg.conf
includes a line 'use-agent'

irrespectively whether using
$ update-alternatives --config pinentry
with
/usr/bin/pinentry-genome3
or
/usr/bin/pinentry-pt

when turning into debug mode
$ killall gpg-agent
$ gpg-agent --debug-level expert --use-standard-socket --daemon /bin/sh

results in immediate return is

gpg-agent[2572]: WARNUNG: "--use-standard-socket" ist eine veraltete Option - sie hat keine Wirkung.
gpg-agent[2572]: enabled debug flags: command cache ipc
gpg-agent[2572]: Es wird auf Socket `/home/psx/.gnupg/S.gpg-agent' gehört
gpg-agent[2573]: gpg-agent (GnuPG) 2.1.11 started

Then, after reboot, when opening an encrypted email debugging reports

"
gpg-agent[3430]: Handhabungsroutine 0xb78fab40 für fd 4 gestartet
gpg-agent[3430]: DBG: chan_4 -> OK Pleased to meet you, process 3500
gpg-agent[3430]: DBG: chan_4 <- RESET
gpg-agent[3430]: DBG: chan_4 -> OK
gpg-agent[3430]: DBG: chan_4 <- OPTION display=:0.0
gpg-agent[3430]: DBG: chan_4 -> OK
gpg-agent[3430]: DBG: chan_4 <- OPTION xauthority=/home/psx/.Xauthority
gpg-agent[3430]: DBG: chan_4 -> OK
gpg-agent[3430]: DBG: chan_4 <- OPTION putenv=DBUS_SESSION_BUS_ADDRESS=unix:abstract=/tmp/dbus-MIY0ixeDzj
gpg-agent[3430]: DBG: chan_4 -> OK
gpg-agent[3430]: DBG: chan_4 <- GETINFO version
gpg-agent[3430]: DBG: chan_4 -> D 2.1.11
gpg-agent[3430]: DBG: chan_4 -> OK
gpg-agent[3430]: DBG: chan_4 <- OPTION allow-pinentry-notify
gpg-agent[3430]: DBG: chan_4 -> OK
gpg-agent[3430]: DBG: chan_4 <- OPTION agent-awareness=2.1.0
gpg-agent[3430]: DBG: chan_4 -> OK
gpg-agent[3430]: DBG: chan_4 <- AGENT_ID
gpg-agent[3430]: DBG: chan_4 -> ERR 67109139 Unbekanntes IPC Kommando <GPG Agent>
gpg-agent[3430]: DBG: chan_4 <- HAVEKEY 23BB0BBB6A2CEF81781A18B963AA2FD08B49A829 9C41A8AB5E132601D444C4E6766854D1DB9B5C4E 5DB16E157CA0EA52F3B6749858BF2F31FC702559 EA0BB2AB2D322AE4096E4EE8B2F37B66107F19DA
gpg-agent[3430]: DBG: chan_4 -> OK
gpg-agent[3430]: DBG: chan_4 <- KEYINFO 23BB0BBB6A2CEF81781A18B963AA2FD08B49A829
gpg-agent[3430]: DBG: agent_get_cache '23BB0BBB6A2CEF81781A18B963AA2FD08B49A829' (mode 2) ...
gpg-agent[3430]: DBG: ... miss
gpg-agent[3430]: DBG: chan_4 -> S KEYINFO 23BB0BBB6A2CEF81781A18B963AA2FD08B49A829 D - - - P - - -
gpg-agent[3430]: DBG: chan_4 -> OK
gpg-agent[3430]: DBG: chan_4 <- KEYINFO 9C41A8AB5E132601D444C4E6766854D1DB9B5C4E
gpg-agent[3430]: DBG: agent_get_cache '9C41A8AB5E132601D444C4E6766854D1DB9B5C4E' (mode 2) ...
gpg-agent[3430]: DBG: ... miss
gpg-agent[3430]: DBG: chan_4 -> S KEYINFO 9C41A8AB5E132601D444C4E6766854D1DB9B5C4E D - - - P - - -
gpg-agent[3430]: DBG: chan_4 -> OK
gpg-agent[3430]: DBG: chan_4 <- KEYINFO 5DB16E157CA0EA52F3B6749858BF2F31FC702559
gpg-agent[3430]: DBG: agent_get_cache '5DB16E157CA0EA52F3B6749858BF2F31FC702559' (mode 2) ...
gpg-agent[3430]: DBG: ... miss
gpg-agent[3430]: DBG: chan_4 -> S KEYINFO 5DB16E157CA0EA52F3B6749858BF2F31FC702559 D - - - P - - -
gpg-agent[3430]: DBG: chan_4 -> OK
gpg-agent[3430]: DBG: chan_4 <- KEYINFO EA0BB2AB2D322AE4096E4EE8B2F37B66107F19DA
gpg-agent[3430]: DBG: agent_get_cache 'EA0BB2AB2D322AE4096E4EE8B2F37B66107F19DA' (mode 2) ...
gpg-agent[3430]: DBG: ... miss
gpg-agent[3430]: DBG: chan_4 -> S KEYINFO EA0BB2AB2D322AE4096E4EE8B2F37B66107F19DA D - - - P - - -
gpg-agent[3430]: DBG: chan_4 -> OK
gpg-agent[3430]: DBG: chan_4 <- HAVEKEY E67E4DD89C576CFE7DE7D7067A1ADD93AFC3F754 8997CD8DEF7825F8F60F09DB251EFD3A9CA46A9F
gpg-agent[3430]: DBG: chan_4 -> OK
gpg-agent[3430]: DBG: chan_4 <- KEYINFO E67E4DD89C576CFE7DE7D7067A1ADD93AFC3F754
gpg-agent[3430]: DBG: agent_get_cache 'E67E4DD89C576CFE7DE7D7067A1ADD93AFC3F754' (mode 2) ...
gpg-agent[3430]: DBG: ... miss
gpg-agent[3430]: DBG: chan_4 -> S KEYINFO E67E4DD89C576CFE7DE7D7067A1ADD93AFC3F754 D - - - P - - -
gpg-agent[3430]: DBG: chan_4 -> OK
gpg-agent[3430]: DBG: chan_4 <- KEYINFO 8997CD8DEF7825F8F60F09DB251EFD3A9CA46A9F
gpg-agent[3430]: DBG: agent_get_cache '8997CD8DEF7825F8F60F09DB251EFD3A9CA46A9F' (mode 2) ...
gpg-agent[3430]: DBG: ... miss
gpg-agent[3430]: DBG: chan_4 -> S KEYINFO 8997CD8DEF7825F8F60F09DB251EFD3A9CA46A9F D - - - P - - -
gpg-agent[3430]: DBG: chan_4 -> OK
gpg-agent[3430]: DBG: chan_4 <- HAVEKEY F44CAD2796AD175C2E810D79654720EDB1ADC654 6512BDD38B69B4E895ED60AD4E8A95148A8D7EB9
gpg-agent[3430]: DBG: chan_4 -> ERR 67108881 Kein geheimer Schlüssel <GPG Agent>
gpg-agent[3430]: DBG: chan_4 <- HAVEKEY ADC53CBFEA0E83DF0C358ABC3F06EBD115EEBFF5 EE829A14B168C592E727E5A2DE3110BE09ECFC18 8133CA937119BC5FD4849E2CFB271924730C57AF 0843B0DE4A82517364B3666026DCC470F8202945 B338AF3872F3C7EDB0132DDAAC91F8F9D12D04A8 D021BBC4CEDB731A96926254792D4B68DCD56186 0D73F49B5AFDE4C40499E14DABA1DD89FB92A391
gpg-agent[3430]: DBG: chan_4 -> ERR 67108881 Kein geheimer Schlüssel <GPG Agent>
gpg-agent[3430]: DBG: chan_4 <- HAVEKEY D9E76BE9ECE8DD29D9DFCB9CF54CB6D50244721B E0935FE2D7829F0CA7580D98F58ECD6EF0A9DE25 036EA59B8B95339F9264F3B5AB90DFC059DB2D5C
gpg-agent[3430]: DBG: chan_4 -> ERR 67108881 Kein geheimer Schlüssel <GPG Agent>
gpg-agent[3430]: DBG: chan_4 <- [eof]
gpg-agent[3430]: Handhabungsroutine 0xb78fab40 für den fd 4 beendet
"

Any help highly appreciated
many thanks