Did you find a solution to this?

I've been having a never-ending series of errors related to missing files since upgrading from 8.3. I'm currently on version 8.4.4 and it feels like I've been doing a lot of copying & editing missing directories & files.

My "fix" for this error was to copy the code from concrete\authentication\concrete\controller.php and update the namespace to

namespace Concrete\Authentication\ExternalConcrete5;

It cleared the error so I can log out and log back in without any errors, but I'm not holding my breath. FWIW I've also tried upgrading to the latest version, currently 8.5.1 but it threw so many errors it didn't seem worth it. :|

Sorry for the late reply, for me updating to the most recent version (that was 8.5.1 at the time) worked. For me concrete5 is full of bugs lately, i don't know if that's the case for everybody, but i find it very annoying.