[d.r.l]

Still modified the code, scheduled the recipe for download, adding the logon and password directly in the GUI as requested.

The code now is:

import re, zipfile, os
from calibre.ptempfile import PersistentTemporaryDirectory
from calibre.ptempfile import PersistentTemporaryFile
from urllib.parse import urlparse, urlsplit
GET_MOBI=False
GET_PDF=True
GET_AUDIO=False
GET_MAGAZIN=False

class TheSkepticalInquirer(BasicNewsRecipe):
    title = u'The Skeptical Inquirer'
    description = 'Investigation of fringe science and paranormal claims.'
    language = 'en' 
    needs_subscription = True 

def build_index(self):
        browser = self.get_browser()

        # find the links (Adjust to your needs!)
        #epublink = browser.find_link(text_regex=re.compile('.*Download ePub.*'))
        #mobilink = browser.find_link(text_regex=re.compile('.*Download Mobi.*'))
        pdflink = browser.find_link(text_regex=re.compile('https://skepticalinquirer.org/archive/'))

        # Cheat calibre's recipe method, as in post from Starsom17
        self.report_progress(0,_('downloading epub'))
        response = browser.follow_link(epublink)
        dir = PersistentTemporaryDirectory()
        epub_file = PersistentTemporaryFile(suffix='.epub',dir=dir)
        epub_file.write(response.read())
        epub_file.close()
        zfile = zipfile.ZipFile(epub_file.name, 'r')
        self.report_progress(0.1,_('extracting epub'))
        zfile.extractall(self.output_dir)
        epub_file.close()
        index = os.path.join(self.output_dir, 'content.opf')
        self.report_progress(0.2,_('epub downloaded and extracted'))


        #
        # Now, download the remaining files
        #
        if (GET_MOBI):
           self.report_progress(0.3,_('downloading mobi'))
           mobi_file = PersistentTemporaryFile(suffix='.mobi',dir=dir)
           browser.back()
           response = browser.follow_link(mobilink)
           mobi_file.write(response.read())
           mobi_file.close()

        if (GET_PDF):
           self.report_progress(0.4,_('downloading pdf'))
           pdf_file = PersistentTemporaryFile(suffix='.pdf',dir=dir)
           browser.back()
           response = browser.follow_link(pdflink)
           pdf_file.write(response.read())
           pdf_file.close()

        # Get all formats into Calibre's database as one single book entry
        self.report_progress(0.6,_('Adding files to Calibre db'))
        cmd = "calibredb add -1 " + dir
        os.system(cmd)

        return index

The error message now is:

Fetch news from The Skeptical Inquirer
Conversion options changed from defaults:
  verbose: 2
  output_profile: 'tablet'
Resolved conversion options
calibre version: 5.13.0
{'asciiize': False,
 'author_sort': None,
 'authors': None,
 'base_font_size': 0,
 'book_producer': None,
 'change_justification': 'original',
 'chapter': None,
 'chapter_mark': 'pagebreak',
 'comments': None,
 'cover': None,
 'debug_pipeline': None,
 'dehyphenate': True,
 'delete_blank_paragraphs': True,
 'disable_font_rescaling': False,
 'dont_download_recipe': False,
 'dont_split_on_page_breaks': True,
 'duplicate_links_in_toc': False,
 'embed_all_fonts': False,
 'embed_font_family': None,
 'enable_heuristics': False,
 'epub_flatten': False,
 'epub_inline_toc': False,
 'epub_toc_at_end': False,
 'epub_version': '2',
 'expand_css': False,
 'extra_css': None,
 'extract_to': None,
 'filter_css': None,
 'fix_indents': True,
 'flow_size': 260,
 'font_size_mapping': None,
 'format_scene_breaks': True,
 'html_unwrap_factor': 0.4,
 'input_encoding': None,
 'input_profile': <calibre.customize.profiles.InputProfile object at 0x0000022086166A30>,
 'insert_blank_line': False,
 'insert_blank_line_size': 0.5,
 'insert_metadata': False,
 'isbn': None,
 'italicize_common_cases': True,
 'keep_ligatures': False,
 'language': None,
 'level1_toc': None,
 'level2_toc': None,
 'level3_toc': None,
 'line_height': 0,
 'linearize_tables': False,
 'lrf': False,
 'margin_bottom': 5.0,
 'margin_left': 5.0,
 'margin_right': 5.0,
 'margin_top': 5.0,
 'markup_chapter_headings': True,
 'max_toc_links': 50,
 'minimum_line_height': 120.0,
 'no_chapters_in_toc': False,
 'no_default_epub_cover': False,
 'no_inline_navbars': False,
 'no_svg_cover': False,
 'output_profile': <calibre.customize.profiles.TabletOutput object at 0x000002208618B370>,
 'page_breaks_before': None,
 'prefer_metadata_cover': False,
 'preserve_cover_aspect_ratio': False,
 'pretty_print': True,
 'pubdate': None,
 'publisher': None,
 'rating': None,
 'read_metadata_from_opf': None,
 'remove_fake_margins': True,
 'remove_first_image': False,
 'remove_paragraph_spacing': False,
 'remove_paragraph_spacing_indent_size': 1.5,
 'renumber_headings': True,
 'replace_scene_breaks': '',
 'search_replace': None,
 'series': None,
 'series_index': None,
 'smarten_punctuation': False,
 'sr1_replace': '',
 'sr1_search': '',
 'sr2_replace': '',
 'sr2_search': '',
 'sr3_replace': '',
 'sr3_search': '',
 'start_reading_at': None,
 'subset_embedded_fonts': False,
 'tags': None,
 'test': False,
 'timestamp': None,
 'title': None,
 'title_sort': None,
 'toc_filter': None,
 'toc_threshold': 6,
 'toc_title': None,
 'transform_css_rules': None,
 'unsmarten_punctuation': False,
 'unwrap_lines': True,
 'use_auto_toc': False,
 'verbose': 2}
InputFormatPlugin: Recipe Input running
Downloading recipe urn: custom:1000
Using user agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.182 Safari/537.36
Traceback (most recent call last):
  File "runpy.py", line 194, in _run_module_as_main
  File "runpy.py", line 87, in _run_code
  File "site.py", line 82, in <module>
  File "site.py", line 77, in main
  File "site.py", line 49, in run_entry_point
  File "calibre\utils\ipc\worker.py", line 216, in main
  File "calibre\gui2\convert\gui_conversion.py", line 34, in gui_convert_recipe
  File "calibre\gui2\convert\gui_conversion.py", line 28, in gui_convert
  File "calibre\ebooks\conversion\plumber.py", line 1105, in run
  File "calibre\customize\conversion.py", line 245, in __call__
  File "calibre\ebooks\conversion\plugins\recipe_input.py", line 139, in convert
  File "calibre\web\feeds\news.py", line 1051, in download
  File "calibre\web\feeds\news.py", line 1228, in build_index
  File "calibre\web\feeds\news.py", line 1659, in parse_feeds
  File "calibre\web\feeds\news.py", line 474, in get_feeds
NotImplementedError

I feel like I'm getting there, yet not...

Thanks for your help!