Thanks for ideas, I did as you proposed, however issue still persists.
I followed all 5 steps. Still same error when trying to download stories.
Did some more tests, results are weird.
1. Tried command line as you suggested in another thread:
"c:\program files\calibre2\calibre-debug" --run-plugin FanFicFare
https://storiesonline.net/s/66637/citizen-miller
Book downloaded without issues into folder (not added to calibre)
2. Tried some random book from royalroad.com with calibre and FFF: download successful!
3. Tried again some other book from SOL: same problem as before.
Debug log:
(Removed log with a bunch of NFSW links. --JimmXinu)