Cannot upload a template to Captive Portal

[sense8]

The system produces errors upon attempting to upload a template to the captive portal. This is the system version:
Versions   OPNsense 18.1.6-amd64
FreeBSD 11.1-RELEASE-p9
OpenSSL 1.0.2o 27 Mar 2018

This is the error produced:

Code: [Select]

Jul 10 10:11:55 OPNsense configd.py: [7301715b-f5a2-4ad2-96d8-4247130b066b] strip user captiveportal web template package and return new base64 encoded content
Jul 10 10:11:55 OPNsense configd.py: [7301715b-f5a2-4ad2-96d8-4247130b066b] Script action failed with Command '/usr/local/opnsense/scripts/OPNsense/CaptivePortal/strip_template.py 'cp_
acd567bd-dcd5-4c4b-a720-8ac46abc075c.tmp'' returned non-zero exit status 1 at Traceback (most recent call last):   File "/usr/local/opnsense/service/modules/processhandler.py",
line 479, in execute     stdout=output_stream, stderr=error_stream)   File "/usr/local/lib/python2.7/subprocess.py", line 186, in check_call raise CalledProcessError(retcode, cmd)
CalledProcessError: Command '/usr/local/opnsense/scripts/OPNsense/CaptivePortal/strip_template.py 'cp_acd567bd-dcd5-4c4b-a720-8ac46abc075c.tmp'' returned non-zero exit status 1


[sense8]

It seems to be the same issue that this guy has:

https://forum.opnsense.org/index.php?topic=5963.msg24791#msg24791

His question remains unanswered after years