wifi on buster on raspi 4b works partly but does hardly connect

[Freek de Kruijf]

# uname -a
Linux bach2 4.19.97-v7l+ #1294 SMP Thu Jan 30 13:21:14 GMT 2020 armv7l GNU/Linux
The system, Raspberry Pi 4B, with the full Buster image, is meant to have a stable wifi connection with my access point. The latest updates are installed.
When booting the system sometimes the in-build wifi interface, wlan0, comes up, mostly not.
The command
# iwlist wlan0 scan gives:
....
Address: 98:9B:CB:9D:F4:39
                    Channel:11
                    Frequency:2.462 GHz (Channel 11)
                    Quality=40/70 Signal level=-70 dBm
                    Encryption key:on
                    ESSID:"xxxxxx"
-70 dBm should be enough for a stable connection
# ifdown wlan0
...
# ifup wlan0
Internet Systems Consortium DHCP Client 4.4.1
Copyright 2004-2018 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

Listening on LPF/wlan0/dc:a6:32:55:12:bf
Sending on LPF/wlan0/dc:a6:32:55:12:bf
Sending on Socket/fallback
DHCPDISCOVER on wlan0 to 255.255.255.255 port 67 interval 7
DHCPDISCOVER on wlan0 to 255.255.255.255 port 67 interval 12
DHCPDISCOVER on wlan0 to 255.255.255.255 port 67 interval 8
DHCPDISCOVER on wlan0 to 255.255.255.255 port 67 interval 15
DHCPDISCOVER on wlan0 to 255.255.255.255 port 67 interval 19
No DHCPOFFERS received.
No working leases in persistent database - sleeping.

So, no connection.
During the above process I gave
# tcpdump -v -i wlan0
Which did not show any output.
Only after finishing the command "ifup wlan0" and some waiting, the tcpdump command started to show output and later a connection came up. But shortly after the status is disassociated.

I can't think of any wrong configuration in interfaces:

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp

auto wlan0
iface wlan0 inet dhcp
wireless-power off

 or wpa_supplicant:

ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1
country=NL

network={
 ssid="XXXXXXX"
 psk="password"
 key_mgmt=WPA-PSK
}

In short unreliable.

[Freek de Kruijf (f-de-kruijf-gmail)]

Continued the analyses using debug output from wpa_supplicant.
Command: wpa_supplicant -c<conf-file> -iwlan0 -d
My first try did not result in a successful association.
After that I did catch the output in a file.
At first the AP to connect to ended in the blacklist, which went on,
but finally the status ended in ASSOCIATED.
However it went on and seems to start all over again.

[Freek de Kruijf (f-de-kruijf-gmail)]

# wpa_supplicant -v
wpa_supplicant v2.8-devel
Copyright (c) 2003-2019, Jouni Malinen <email address hidden> and contributors