Lab Manager only ever made one hop if you import from SMB ... the ESX Classic host would mount the SMB share directly, then perform an import. That's really simple compared to vCloud Director.
vCloud uses a Java client to upload from your local through one of the vCloud Director cells, placing the file into the $VCLOUD_HOME/data/transfer folder.
Please ensure that your transfer folder has enough free space to accomodate the entire OVF upload.
Once there and received in it's entirety, the Cell will then upload the VM to an ESX host which has access to the appropriate destination Provider/Org vDC/Datastore.
"Transfer session timeout"
This is if you pause an upload .... it's only going to wait so long for you to click the resume button.
the vCloud Container debug log should show a reason for failure, around the time the UI stated as such.
one of the terms I commonly search by is "Caused by: java"
This line is typically part of a Java Stack Trace ... it'll be preceeded by a number of lines that are indented and start with "at ..."
The line immediately above all the lines starting with "at" should be an ERROR state.
However, not all ERROR states will have a "Caused by" line.