I've had the same issues, and here's how I was able to solve it (no errors, everything seems to be working fine so far):

(mind you, I'm still new to Piwigo, so could be a better way)
1. Installed the VideoJS plugin through Piwigo admin panel
2. Deactivated the videojs plugin.
3. Downloaded the videojs 2.8b release from the github.
4. replaced 2 files (and copied permissions) in the plugin directory with those from the downloaded release, example (change your directory paths as needed):

sudo cd /var/www/html/piwigo/plugins/piwigo-videojs/admin
sudo mv admin_sync.php admin_sync.php.BK
sudo cp /mnt/e/downloads/piwigo-videojs-2.8.b/admin/admin_sync.php .
sudo chown --reference=admin_sync.php.BK admin_sync.php
sudo chmod --reference=admin_sync.php.BK admin_sync.php

cd /var/www/html/piwigo/plugins/piwigo-videojs/include
sudo mv function_dependencies.php function_dependencies.php.BK
sudo cp /mnt/e/downloads/piwigo-videojs-2.8.b/include/function_dependencies.php .
sudo chmod --reference=function_dependencies.php.BK function_dependencies.php
sudo chown --reference=function_dependencies.php.BK function_dependencies.php

So basically I've backed up the 2 error-prone 2.9b files and replaced them with those from 2.8b. No idea what are the differences, but errors are gone. So far syncs in VideoJS and Batch Manager work with no errors. I'll see if I get hidden errors along the line somewhere.

Oh, and don't forget to re-enable the videojs plugin from the piwigo admin panel.