Most likely you incorrectly indicate the server Aetitle as the defiant: the -aet option should be replaced by the -aec option. You also need to specify the Aetitle of your device (-aet):
dcmsend +r -aet YOU_AET -aec DCM4CHEE <virtualbox IP> 11112 --scan-directories ./
You can send all the files in the nested folders by another command - storescu. For example:
storescu -ll warn +r -aet MY_AET -aec DCM4CHEE <IP host dcm4chee-arc> 11112 +sd /home/dicom/dcmfiles