I had a documentation site generated using hugo. It was working fine until I upgraded to ubuntu 18.04. I guess that also upgraded hugo.
Now index.html is generated, with links to sub directories but the html for the subdirectories are not generated.
No configuration or code or layout has changed. Just hugo version update. Current version is 0.40.1.
I am using Material Docs theme. Can there be compatibility issues of a theme with hugo version?
Yesterday i had a similar problem under macOS with the same hugo version. I tried to generate in a subdirectory a list, which shows all markdown files in there. Every time i got an 404 error, when i navigate to my subdirectory url. I thought the whole time that my code was wrong (beginner) but after some hours i found a work around. When i moved my files out of the subdirectory and renamed the _index.md to index.md, i saw my single template result. After renaming back to _index.md, i saw first my list template result. And when i now moved file for file back, hugo refreshed the site file for file.
The strange thing was, that after stopping and restarting the hugo server i got a 404 error about the same folders, which worked before. And the hugo command render also no subdirectories. I think, your and mine problem belongs together and there seems to be a bug inside hugo.