Unable to import a VSDX file

[CyberTech71]

When attempting to import a VSDX file into draw.io the spinner keeps on spinning and nothing happens - no error, no progress, just nothing.
CPU for the draw.io processes is being consumed at a reasonable level and changing so it looks like activity is happening but after waiting an hour, the VSDX file is still waiting to import.
Note: the VSDX file uses a custom shape template in the form of a VSSX file - maybe this is the problem?