I was having some weird problems with my migrated site.
First off, I was getting a number of
PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 81 bytes) in /home/drupal/vendor/twig/twig/lib/Twig/Error.php on line 214
or similar errors on bogus urls to the album photos module and to certain attack probes. These attack probes were urls like
Secondly, the error page for other bogus urls was not showing a proper error message. Instead it showed the front page, although with some of the contents not showing properly due to faulty paths.
It turned out that both of these issues were because either the migration process or the initial installation set the custom error pages to / instead of being empty (to use the default error messages). Once I emptied the fields, both errors were resolved.
Once I was done, I decided to play about with my printable module configuration, which promptly broke. It started throwing
Uncaught PHP Exception Drupal\\Component\\Plugin\\Exception\\PluginNotFoundException: "The "" plugin does not exist." at /home/drupal/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php line 52
errors. After a bit of googling, I found that a number of people were getting errors because of configuration problems. The configuration for the printable module was fine after the initial installation, but the configuration menus came up with bogus data, which only caused problems when saved. There seems to be issues in in Drupal 8 configuration system with reading and/or saving data.