Drupal 8 file upload issue
When attempting to create new content that uses an H5P element, on "save" I get this error:
This value should not be null.
and the file upload input is highlighted in red. The file was uploaded and unzipped to the sites/files/h5p/temp directory, where it was placed in an h5p-[13-digit-alphanumeric] directory before the process ABENDed.
On page refresh (F5), after the form resubmits these error messages are finally exposed:
The file could not be saved because the upload did not complete. File upload error. Could not move uploaded file. The uploaded file doesn't have the required '.h5p' extension
upload_max_filesize and post_max_size were both reset to 128M, to no effect. The two packages listed below, when unzipped to the temp directory, are 2.7M and 2.8M respectively.
H5P v8.x-1.0-rc7 (enabled)
H5P Editor v8.x-1.0-rc7 (enabled)
Firefox, latest version, desktop, no console errors. Mostly ditto with Google Chrome, latest version (no additional errors on refresh)
No PHP errors to the apache2 error.log file (which PHP logs to) or displayed on screen (xdebug is installed)
Nothing meaningful in Watchdog (says Notice:The file public://h5p/temp/h5p-5b35127dd907e/multiple-choice-713.h5p was not deleted because it does not exist.)
No config or other issues reported under Reports > Status Report (file system is writable)
I am running Drupal on Ubuntu 18.04, with Ubuntu as a Guest VM under VirtualBox v5.2.12, with Windows 10 as Host. vbox extensions and guest additions are installed.
I am logged into drupal as user 1 (root)
This looks like a really nasty bug, but where exactly? Or am I doing something wrong even though I followed the instructions at https://h5p.org/documentation/setup/drupal8 ?