Thank you @jmiranda, suggested post helped me a lot.
The issues I was experiencing are the ones described in the thread https://community.openboxes.com/t/internal-server-error-cannot-get-property-inputstream-on-null-object/1045.
As indicated in the thread, the problem was in the machine’s local configuration; in my case:
#$ localectl
System Locale: LANG=es_ES.UTF-8
VC Keymap: es
X11 Layout: es
After changing it to “en_US.UTF-8”, the application worked as expected:
#$ localectl set-locale "en_US.UTF-8"
#$ localectl
System Locale: LANG=en_US.UTF-8
VC Keymap: es
X11 Layout: es