While upgrading from OPNsense 23,7 to 23.7.1_3 the following error did appear:
An API exception occurred:
/usr/local/opnsense/mvc/app/controllers/OPNsense/Core/Api/FirmwareController.php:41: Class "OPNsense\Base\ApiMutableModelControllerBase" not found
The upgrade process did continue and OPNsense seems to work just fine after the reboot
So no idea what this message means but just mentioned it.
This can happen when the package manager replaces the file while the server tries to serve it (since we poll the firmware page for progress), but it's not a problem afterwards.
Cheers,
Franco