Just in case this helps anyone else- I wasted a lot of time on this one this morning. I appreciate ovftools is a VMWare product...

I was trying to convert an exported ovf vm folder to .ova as a single file for import into AHV.

I was just getting a generic error- file "" could not be opened:

trying to test or verify I got similar:

c:\Program Files\VMware\VMware OVF Tool>ovftool --verifyOnly --machineOutput V:\Grid-lic-server\Grid-lic-server.vmx

ERROR

+ <Errors>

+ <Error>

+ <Type>ovftool.io.file.open</Type>

+ <LocalizedMsg>

+ Failed to open file: V:\Grid-lic-server\Grid-lic-server.vmx

+ </LocalizedMsg>

+ <Arg>

+ V:\Grid-lic-server\Grid-lic-server.vmx

+ </Arg>

+ </Error>

+ </Errors>

RESULT

+ ERROR

Not very illuminating.

I tried setting a log output and for next to nothing useful even in verbose mode:

However:

When I navigated to the local directory and ran the same export / convert command where the .vmx file was located I got:

C:\Program Files\VMware\VMware OVF Tool>ovftool D:\Grid-lic-server\Grid-lic-server.vmx D:\ova\grid-lic-server.ova

Opening VMX source: D:\Grid-lic-server\Grid-lic-server.vmx

Error: File (D:\Grid-lic-server\/vmfs/volumes/54de0796-0c4fc176-6dc6-c4346ba9686c/installers&isos/microsoft_images/windows_server_2012_R2_vol_lic/SW_DVD9_Windows_Svr_Std_and_DataCtr_2012_R2_64Bit_English_-3_MLF_X19-53588.ISO) could not be found.

Completed with errors

Now this is a bit more like it! : I can see I had an image file attached that ovftool now cant find. 

I simply opened it up in player, removed the CD volume in settings and then saved it.

I then reran the command and it wrote out the single .ova file as I expected.

Hope this helps someone else.