Strange error message when trying to resize a VirtualBox VDI

I'm running

vboxmanage modifyhd /path/to/my.vdi --resize 35000

and getting:

VBoxManage: error: Cannot register the hard disk '/path/to/my.vdi' {8494ccfd-bc55-4826-abdd-773f98afd32c} because a hard disk '/path/to/my.vdi' with UUID {8494ccfd-bc55-4826-abdd-773f98afd32c} already exists
VBoxManage: error: Details: code NS_ERROR_INVALID_ARG (0x80070057), component VirtualBoxWrap, interface IVirtualBox, callee nsISupports
VBoxManage: error: Context: "OpenMedium(Bstr(pszFilenameOrUuid).raw(), enmDevType, enmAccessMode, fForceNewUuidOnOpen, pMedium.asOutParam())" at line 191 of file VBoxManageDisk.cpp

Well, of course the disk exists - I'm trying to resize an existing disk. So why am I getting this error message? And - how should I actually resize my VDI?

I'm using VirtualBox 6.0.10 on Devuan Beowulf (= Debian Buster without systemd).