err=23 when updating textboxes - python-3.x

Question

I'm getting the following error when updating text boxes:
load glyph failed err=23 face=0x0, glyph=19
All of the lines responsible for updating the text boxes look like this the par:
self.lnePer1_temp.setText(str(values_s[0]))# + " °C")
The method which aggregates these methods is called like this:
def timer_handler(self):
self.pressedSearchDevices()
if self.grbKeys1.isEnabled():
self.pressedKey1_bat()
if self.grbKeys2.isEnabled():
self.pressedKey2_bat()
if self.grbPer1.isEnabled():
self.pressedPer1_read_all()
if self.grbPer2.isEnabled():
self.pressedPer2_read_all()
threading.Timer(60, lambda: self.timer_handler()).start()
Basically it checks periodically whether some devices are available and updates the textboxes accordingly. When commented out - everything works fine. The full method responsible for updating the textboxes is:
def pressedPer2_read_all(self):
call('./reset_ble.sh', shell=True)
cmd = "sudo node Per_read_all.js " + Per_2.lower()
output = 'None'
with Popen(cmd, shell=True, stdout=PIPE, preexec_fn=os.setsid) as process:
try:
output = process.communicate(timeout=5)[0]
except TimeoutExpired:
os.killpg(process.pid, signal.SIGINT) # send signal to the process group
output = process.communicate()[0]
print(output)
output_s = output.decode("utf-8")
values_s = output_s.split('\n')
for s in values_s: print(s + " °C")
if len(values_s) == 7:
self.lnePer2_temp.setText(str(values_s[0]))# + " °C")
self.lnePer2_dig.setText(str(values_s[1]))
self.lnePer2_light.setText(str(values_s[2]))# + " %")
self.lnePer2_hum.setText(str(values_s[3]))# + " %")
self.lnePer2_alert.setText(str(values_s[4]))
self.lnePer2_but.setText(str(values_s[5]))
else:
self.lnePer2_temp.setText(" °C")
self.lnePer2_dig.setText(" ")
self.lnePer2_light.setText(" %")
self.lnePer2_hum.setText(" %")
self.lnePer2_alert.setText(" ")
self.lnePer2_but.setText(" ")
When replacing the if-else statement with only the else statement - everything works fine, so I'm assuming that the error lies in the change of values or something like that. Similar error messages link to Qt, so I'm also assuming that the fault lies somewhere there.
The script is running on a Raspberry Pi Zero, the Python Version used is Python 3.4.2, the PyQt5 version is 5.3.2.