Not a 3.06a issue, just started this afternoon (3/30). May be a change in the lookup query at discogs.com (which still shows covers in its internal queries).
They're currently playing with their image URLs, which disturbs the image caching proxy I'm using. I've already adapted the proxy to deal with most changes, but it seems not all.
I've made some adjustments here and it should be working again.
Yes. Back again. Thanks.
Looks like discogs has changed again ... many covers missing from the retrieval ... e.g. 7594371
This time the server ran out of disc space 
I've now increased the volume for the cover cache to 600 GB and it should be back and working again.
Confirmed. It's working again. Thanks.
Looks like covers are missing again.
Thx
They're back. Thanks if thanks are due to the mp3tag team.
And they're gone again... looks like a site problem.
I've fixed it yesterday morning and everything should be back to normal. Thanks for pointing!