Migration from OC 10.6 fails with unhandel exception error

Iā€™m attempting to migrate from OC 10.6 to Nextcloud 21.0.2 and have followed the Upgrade Manually process and got as fare as the ā€œsudo -u www-data php occ upgradeā€ step and received the following error information:
ā€œAn unhandled exception has been thrown:
Error: Interface ā€˜OCP\Files\External\Config\IBackendProviderā€™ not found in /var/www/owncloud/apps/files_external/lib/AppInfo/Application.php:40
Stack trace:
#0 /var/www/nextcloud/lib/composer/composer/ClassLoader.php(478): include()
#1 /var/www/nextcloud/lib/composer/composer/ClassLoader.php(346): Composer\Autoload\includeFile()
#2 [internal function]: Composer\Autoload\ClassLoader->loadClass()
#3 [internal function]: spl_autoload_call()
#4 /var/www/nextcloud/lib/private/AppFramework/Bootstrap/Coordinator.php(116): class_exists()
#5 /var/www/nextcloud/lib/private/AppFramework/Bootstrap/Coordinator.php(82): OC\AppFramework\Bootstrap\Coordinator->registerApps()
#6 /var/www/nextcloud/lib/base.php(634): OC\AppFramework\Bootstrap\Coordinator->runInitialRegistration()
#7 /var/www/nextcloud/lib/base.php(1076): OC::init()
#8 /var/www/nextcloud/console.php(49): require_once(ā€™/var/www/nextclā€¦ā€™)
#9 /var/www/nextcloud/occ(11): require_once(ā€™/var/www/nextclā€¦ā€™)ā€

This does not appear to be similar to any of the other posts I have perused, so why am I so special??
Can anyone help??

Hi there, did you try migrating to NC20.0.10? Migrating directly to NC21 isnā€™t supported, afaik.
See Migrating from ownCloud ā€” Nextcloud latest Administration Manual latest documentation

Ah I see, but that is not clear from that page. It says 20.0.x ( but at least 20.0.5) and I inferred from that, ā€œthe higher the betterā€. Iā€™ll give 20.0.5 a go and report back.
Thanks.

Well, I tried migrating to NC 20.0.10 as suggested, but the error is exactly the same. I canā€™t help thinking that it has something to do with copying the owncloud config.php file and plonking it into /var/www/nextcloud/config ? I have also left the /var/www/owncloud directory in situ and not deleted it as per the Nextcloud upgrade instructions, as if I delete it, apache2 will not restart due to the directory mentioned in /etc/apache2/sites-available and sites-enabled not being available.

I have created and enabled a nextcloud site in Apache2, reloaded Apache2 and it has made no difference. If I rename the owncloud directory to owncloud.old, I get the following:
ā€œApp directory ā€œ/var/www/owncloud/appsā€ not found! Please put the Nextcloud apps folder in the Nextcloud folder or the folder above. You can also configure the location in the config.php file.ā€

Any ideasā€¦ anyone???

OK, Iā€™ve been poking around a bit and it is clear that I am not understanding the ā€œUpgrade Manuallyā€ process when used in the context of migrating from Owncloud. I copied the /var/www/owncloud/config/config.php file and placed it in /var/www/nextcloud/config. When I ran the upgrade, I got the above error.
I then modified the config.php to use the nextcloud paths for the apps and apps-external directories and am now getting a different error:
An unhandled exceptionā€¦
Exception: Environment not properly prepared. in /var/www/nextcloud.lib/private/Console/Application.php:168
Stack trace:
#0 /var/www/nextcloud/console.php(99): OC\Console\Application->loadCommands()
#1 /var/www/nextcloud/occ/(11): require once(ā€˜var/www/nextclā€¦ā€™)
#2 {main}

I hope this will shed some light on my issue.

Well, Iā€™ve sorted it! The machine is now functioning at Nextcloud 20.0.10. The problem was that although I had ā€œcorrectedā€ the environment by adding the apps and apps-external folders, I had not corrected the permissions to make them accessible to www-data.
I had already modified the config.php file to 1.) Change the version from Owncloud 10.6 to 10.5 and 2.) Use the Nextcloud apps and apps-external directories which seems to be a required step in the migration process.