"The Most Common Drupal 7 to Drupal 8 Migration Issues AND THEIR FIXES" would have been the longer version of this post's title. It's a fact now: you can no longer escape Drupal 8's “influence”, your job as a Drupal developer implies regular Drupal 7 to Drupal 8 migration "marathons". And since everyone around you's overly excited about Drupal 8's purely innovative features, about its irresistibly cutting-edge capabilities (and where do you add that these include a Migration module in its core, as well), you suffer major “shock” discovering that, well, moving websites to Drupal 8 is not always a “boringly” smooth and event-less process.
There are cases when you might grow paranoiac, after several “Page not found” errors, thinking that the whole universe must be against you or that the team of developers “responsible” for the last version of Drupal are testing you (both your skills as a developer, your patience and your “mystery solving skills” ) to solve this hard nut to crack type of a "migration puzzle" (joking).
Well, now let's get to the heart of the matter, shall we? Let's get all these Drupal 7 to Drupal 8 migration issues exposed so that you can pin them up on your "black list" and avoid them whenever you need to complete a D7 to D8 migration process.
1. Oops! My Video Field Content and Configuration Have Not Migrated!
No need to get discouraged when you're facing with this quite common migration problem!
One possible solution (for you know how Drupal works: there's a whole “set” of possible solutions to each problem that the CMS might be challenging you with) to it: create a video field in the YouTube Video module. Then, rely on your script for migrating your video content to Drupal 8.
2. Link Field Without Contents: One of the Popular Drupal 7 to Drupal 8 Migration Issues
It's due to a field configuration mismatching!
What do you do in this case? You roll up your sleeves and do some “manual work” (when you've got to, you've got to, no matter how “automated” the whole Drupal 7 to Drupal 8 migration process is rumored to be!).
In other words: you just need to reconfigure your field settings, to write a script in order to migrate your content to your Drupal 8 website. Remember to save your internal and your external link differently when you save your link through the your script!
3. “Full HTML”, “Filtered HTML” and Other Filter Formats Haven't Migrated
This is no reason for you to drop down the whole Drupal 7 to Drupal 8 process, either!
You can always adopt a “create a new custom script” method! It's the solution that will help you set the right format. And thus to fix another one of the quite "popular" Drupal 7 to Drupal 8 migration issues, as well.
Also, you should know (lest you should panic witnessing this) that: the filter format will be set to “null” in the first place, showing only an empty string!
4. Content Won't Show on My Drupal 8 Site: “Page Not Found” Error
And since I've mentioned something about the “several solutions for one Drupal problem” schema, you're looking at one right now! Not showing your migrated content and replacing it with an alarming “Page not found” alert message is one issue with multiple possible solutions. And one of the most common Drupal 7 to Drupal 8 migration issues, as well!
a. It might be a more than common “I forgot to copy the .htaccess file from my Drupal 7 website” scenario. No need to be embarrassed, it happens even to the most experienced Drupal developers, as well!
Especially since it's hidden and, therefore, very easy to get overlooked during the migration “marathon” with all its implied steps to take, with all the tiny details to pay attention to.
Therefore, this might be It, the “culprit” behind your “Page not found” error page. Your .htaccess file “left behind” might still be linking to your Drupal 7's site's old URLs.
b. Your Drupal website's not running on a “standard” cache/proxy server, but on an nginx and you haven't used the correct way of configuring it for a Drupal 8 installation.
Which leads to problems in getting Clean URLs and Image Styles. In this respect, you might want to delve into this helpful info here: Nginx config for Drupal 8.
c. It's your server (again) that might either need a bit more “attention”, meaning a particular specification for supporting Clean URLs or which simply does not support them. Luckily, in Drupal 8 these (Clean URLs) get enabled by default, yet you still need to enable a rewrite module on your server for that!
d. It might as well be a “migrating to a new Drupal codebase version scenario” and in this case you need you update your DB (via update.php or drush updb). Running these updates will also solve all the cache clearing issues, as well (which might, in fact, be the ones “orchestraing” the whole “page not found conspiracy” against you).
Just a migration piece of advice: ensure that you have all the proper .htaccess rewrite rules in lace prior to restoring your database to your live Drupal 8 website. This advice applies if you have clean URLs on your website.
5. Both Field and Content Migrated, But Without Field Configurations
Can you guess the solution for this problem?
Well, going manual is, again, the most reliable method for fixing it. Manually configure your settings in this case!