Well, I`ve allready expected that. Funfact:
pi@pihole:~ $ curl -sSL https://install.pi-hole.net | bash
curl: (6) Could not resolve host: install.pi-hole.net
The local nameserverentry is allready poiting to quad-9. Have to figure out what`s wrong.
Thanks for you help anyway 
Does dmesg | grep voltage return anything ?
Is your /etc/resolv.conf pointing at 127.0.0.1 (if yes, change it to a public one, 1.1.1.1 or 8.8.8.8 for the command to initially work)?
Allready solved that. My entry wasn't persitant. At the moment, the problem seems to go deeper within the system. The setup-script detected the existing installation, asked to repair without success. Even a fresh install with new config (which run just fine within the script) doesn't fix anything. I think I just save time if i start installing a fresh install just now
I have below list of 0 files:
pi@noads:~ $ sudo find /{opt,usr,bin} -size 0
/usr/lib/python3.5/urllib/__init__.py
/usr/lib/python3.5/email/mime/__init__.py
/usr/lib/python3.5/pydoc_data/__init__.py
/usr/lib/arm-linux-gnueabihf/perl5/5.24/auto/Params/Validate/XS/XS.bs
/usr/lib/arm-linux-gnueabihf/perl5/5.24/auto/Params/Classify/Classify.bs
/usr/lib/php/7.0/sapi/cli
/usr/lib/php/7.0/sapi/cgi
/usr/lib/python2.7/dist-packages/html5lib/filters/__init__.py
/usr/lib/python2.7/dist-packages/pkg_resources/_vendor/__init__.py
/usr/lib/python2.7/dist-packages/RPi/__init__.py
/usr/lib/python2.7/dist-packages/lxml/includes/__init__.py
/usr/lib/python2.7/email/mime/__init__.py
/usr/lib/python2.7/pydoc_data/__init__.py
/usr/share/php5/sapi/cli
/usr/share/php5/sapi/cgi
/usr/share/mime/icons
/usr/share/xml/iso-codes/iso_3166-3.xml
/usr/share/python/debpython/__init__.py
/usr/share/X11/locale/el_GR.UTF-8/XI18N_OBJS
/usr/share/X11/locale/cs_CZ.UTF-8/XI18N_OBJS
/usr/share/X11/locale/am_ET.UTF-8/XI18N_OBJS
/usr/share/X11/locale/fi_FI.UTF-8/XI18N_OBJS
/usr/share/doc/openvpn/examples/sample-keys/sample-ca/secp256k1.pem
/usr/share/doc/libregexp-assemble-perl/examples/hostmatch/error.canonical
If have many more, yes reinstall and this:
Side note: Does vcgencmd still show voltage issues?
Do you mean below ?
vcgencmd get_config int
It looks like it does have over_voltage_avs but no under voltage.
EDIT:
pi@phb5:~ $ vcgencmd get_config int
aphy_params_current=819
arm_freq=900
audio_pwm_mode=514
config_hdmi_boost=5
disable_auto_turbo=1
disable_commandline_tags=2
disable_l2cache=1
display_hdmi_rotate=-1
display_lcd_rotate=-1
dphy_params_current=547
enable_uart=1
force_eeprom_read=1
force_pwm_open=1
framebuffer_ignore_alpha=1
framebuffer_swap=1
init_uart_clock=0x2dc6c00
lcd_framerate=60
over_voltage_avs=0x1b774
pause_burst_frames=1
program_serial_random=1
sdram_freq=450
hdmi_force_cec_address:0=65535
hdmi_force_cec_address:1=65535
hdmi_pixel_freq_limit:0=0x9a7ec80
hdmi_pixel_freq_limit:1=0x9a7ec80
vcgencmd get_throttled
EDIT: was just bit too late 
Aha this one:
https://www.raspberrypi.org/documentation/raspbian/applications/vcgencmd.md
pi@phb5:~ $ vcgencmd get_throttled int
throttled=0x0
Yep
Looks like this
Where that 0x00050005 is the actual code/type of throttling (if any).
Not running buster on this and I don't know how (if) it would show ...
0x00050005 hex = 1010000000000000101 bin
So thats bits 0, 2, 16 & 18:
Bit Meaning
0 Under-voltage detected
1 Arm frequency capped
2 Currently throttled
3 Soft temperature limit active
16 Under-voltage has occurred
17 Arm frequency capping has occurred
18 Throttling has occurred
19 Soft temperature limit has occurred