Error: Class 'CRM_Upgrade_Incremental_General' not found in Civi\Install\Requirements->checkMysqlVersion()

Hi, I am getting this error message with Civi installed on a Drupal 8 site:

Error: Class 'CRM_Upgrade_Incremental_General' not found in Civi\Install\Requirements->checkMysqlVersion() (line 294 of [path]/vendor/civicrm/civicrm-core/Civi/Install/Requirements.php)

I'm currently on 5.26.2. The problem happened a couole of upgrades back - sorry I cannot remember what versions I was upgrading from and too, but it is kept up to date.

The problem only really manifests when I run the Drupal update.php script (essential after upgrading core and modules), plus also on other rare occasions like once when I had to rebuild the permissions.

I have checked and the CRM_Upgrade_Incremental_General class is there, and the file permissions look ok, so I am guessing the problem is with the class loader, but I do not have any experience of how this works. I would be greatful if someone could give me a tip on how to fix this?

I can get round it for now by putting a 'return' statement at the beginning of the relevant function in the Requirements.php file, so can run the upgrade script, so it is not urgent.