Is this happening for all placeholders or just some?

Any errors/warnings on EV server itself?

Do you see the request reach the EV server in IIS logs?  Replicate issue and check these.  Should see a line that contains download.asp.

Have you more than one file server does this work OK?

Can you open the same item OK via Archive Explorer?

Something else to try is to disable or remove the Enhanced Security settings because you have to add the site to the Vault Service Account profile, not the account that your accessing it from.

Hmm. It has stopped happening.
If it re-occurs I will follow the advise posted here.

Thanks,
Gilly