Upgrading Drupal

After uploading the new files calling update.php i got following message:

Ein AJAX-HTTP-Fehler ist aufgetreten.
HTTP-Rückgabe-Code: 500
Im Folgenden finden Sie Debugging-Informationen.
Pfad: /update.php/start?id=197&op=do_nojs&op=do
Statustext: Internal Server Error
Antworttext:

The link to the error message doesnt work: i got only a blank page

Follogwing amin-pages didnt work:
/admin/config
/admin/reports/status
/admin/modules/uninstall

I recently updated Drush and after doing so I can no longer run Drush Update. Everytime I try I get the error:

Update information last refreshed: Never
No code updates available.

I removed and reinstalled drush. I manually updated from Drupal Core 8.0.1 to 8.0.3. Still no change. When I run the Available Updates report I get the error

Failed to get available update data for 8 projects.

I have rebuilt the cache using drush cache-clear and drush cache-rebuild.

Output from drush status:

I am logged in as user 1. When I run update.php it shows the Continue button. When I hit the Continue button it returns to Overview without the dialogue and shows links to Front Page and Administration. No error messages are shown. The Reports =>Recent log messages shows access denied with User as Anonymous (unverified). I have tried clearing caches, logging in and out under both www. and without www. with the same results each time.

I have an active D7 site with a dozen + content types and hundreds of nodes, many cross referenced to each other. If I perform an initial migration, that will give me a baseline content dump to start re-configuring and re-theming from. However I am expecting that it will take quite some time to get all the pieces working together properly. During this period the production site will have had many updates.
Is there any workflow/technique that would allow for a secondary migration/overwrite/dump/merge-purge that could deal with the changes between the old/new sites?

Hey,

So I updated from 8.0.1 to 8.0.2, deleted the core, vendor, and all the files and then copied the new versions over to my public html file. On the site status report it says 8.0.2

Here is what I'm not sure about.

In my log messages I've got 6 messages that say:

DrupalKernel . . . Container cannot be saved to cache.

The severity is listed as a "notice"

And two that say:

I'm having trouble getting the files table to migrate on a Drupal 6 site to Drupal 8.

I've tried it through the UI and command line with Drush tools. The migrate works - mostly - but the files table (and physical files) never migrate. I really don't need the physical files to transfer - I can do that myself - I just need the table, for obvious reasons.

I'm entering a directory on the same server as the file locations:
/var/www/folder.name
Which contains the files in the same directory structure as the live site.