Craft 3 Controller giving 404
Craft 3 action routes are snake-case rather than camelCase thus your route needs to be plugin-handle/controller-handle/function-name instead of pluginHandle/controllerName/functionName

Your Controllers namespace must be prefix\namespace\name\controllers so in your case it could be kr37\drycalendar\controllers not Craft.

Furthermore you don't need to include your plugin name to the classname anymore. Your controller name can be

CalendarOccurrencesFieldController

Your action will then be drycalendar/calendar-occurrences-field/generate-ajax-mini-calendar

Keep in mind you'll have to create routes of you don't use the action parameter in your request. Currently you are trying to access your controller via cp/site route, not via action route