Body, files, and paths problems

Question

I was suggested so many times to migrate my old project from D6, so I finally decided to jump into this. And as it always happens with Drupal, things never work as expected at first attempt.

The old Drupal 6 site has 180 modules (I've switched off a vast majority of them), 100K users, about 3K nodes (within 4 content types), 170 taxonomy terms, more than 5K different node relationships, 34K comments, 7K path aliases, 6K user pictures, 5K files (most of them attached to nodes through file fields) and some other stuff.

The new Drupal 8 site was installed with composer create-project drupal-composer/drupal-project:8.x-dev <FOLDER> --stability dev --no-interaction. I also installed all the suggested migration modules and ran the migration process through Drush. The Drush launcher version is 0.6.0 and Drush is version 9.6.0.

drush migrate-import --all command didn't go well, because it found some missing modules, which are not ported to D8 yet, so my only choice was to use something like drush migrate-import upgrade_d6_taxonomy_term --feedback="100 items" for each migration.

Data was ported, but with some problems.

Files

Some files are not attached to nodes, as they should, especially node images.

Moreover, on /admin/content/files page many of them appear with strange strings, instead of names and are linked to empty files: https://new.site.com/sites/default/files/sliwrunepeswothobrucohaphigeluwruj. All these files are actually empty.

As I understood, all of them are node attached images, used as cover image, supposed to represent an article on taxonomy or views pages.

Body field

It is also ported but with two issues:

1) All HTML tags are converted to <p> tags. E.g. if there was H2 heading, it is <p> tag now. The same thing for <li>, images, youtube embedded videos... all inline tags are deleted completely with their contents or replaced by paragraphs. Class names were wiped as well.

Full HTML is enabled for body fileds on both sites.

2) Body is not showing up, when I view a node. If I go to edit - it is in right place, I can even cahnge it. But on node view <div>, which should be containing body, is empty.

I've googeled it, and some say that they changed site default language or edited body teaser, because it cannot be null... but all these things are fine in my case - site language is the same, as it was in D6 version and node teasers are not empty.

Paths

All 7243 path redirect entries are migrated without errors. I can see all of them on /admin/config/search/path page of new site. However, all node urls are default (node/1234) and if I click on url alias it redirects me back to node/whatever-number.

Also on migrated node, path field is empty.

Creation of a new node doesn't have any of described problems, everything works fine.