I'm having an odd error similar to this:
I'm running WAMP on Windows 10, with Processwire, and get an internal server error trying to access either the front end or admin.
Checking my logs I find:
Error: Class 'ProcessWire\PaymentModule' not found (line 3 of ...\processwire\site\modules\PaymentInvoice\PaymentInvoice.module)
The PaymentModule classs definitely exists, and hasn't been modified, and is namespaced.
The other thing I've noticed, is in the modules.txt log