I had to delete my media to get the artwork back on resync but it seems to be working now. Thanks.

Just a note:
after you install build 594, please go to Options->Check for new/changed media, after that try to resync. This should force repairing the artworks.

Thank you for the log. I have found the cause. It will be fixed in next build. Tracked here:
http://www.ventismedia.com/mantis/view.php?id=13262

Debug log NCRJ0H6DNL submitted using build 593 after the issue occured again. DB coming to you via PM shortly.

You are right. It really looks like the path is incorrectly synced and that is why the black artwork is shown. Unfortunately, your log was not sent - probably due to connection error. I cannot find it.

The issue 13122 occurs only on devices with external SD card and Nexus 6 doesn't have it. So it is quite strange. Could you please try latest build 593. There are many fixes related to storing files.

Could you please try it again and if it fails, please send another log.. it would be also helpful to have your DB. Could you send it to me e.g. via PM ? It is located in /MediaMonkey/files/mmstore.db

Further, if I play one of the tracks that's missing artwork then MMA will show a black square instead of the MediaMonkey logo that's usually shown when there's no artwork, and the widget shows the artwork from the previous track.

Happened again today but this time it seems to have been directly related to WiFi sync. Sequence of events as follows:
1. All artworks were present.
2. WiFi sync. Then all music and audiobook artworks were immediately missing, as well as about half of podcasts.
3. WiFi sync again. This time music and audiobook artworks were still missing but the podcasts artworks that were missing are now present and those that were present are now missing.

Submitted debug log U5CW7WSSJ4.

I ran a third WiFi sync after sending the debug log and now most, but not all, artworks are back.

No, I don't have any apps like that.

I have seen a message that might have been about the database flash on screen the last time the problem occured though. It was too brief to read so just in case it is a database problem I've tried deleting the entire /MediaMonkey/ folder and starting from scratch. Now I'm waiting for the initial sync and we'll see if it happens again with a new database.

Do you use any Cleaning App like CleanMaster?

In build 592 all files in the /MediaMonkey/artworks folder are being deleted at some point. If I delete all tracks and do a WiFi sync then the artwork files are written and all tracks show the artwork within MMA. Then after about half a day I can open MMA and find that suddenly no tracks have artwork and the files in the artworks folder have been deleted.

I'm guessing that it's due to changes made during http://www.ventismedia.com/mantis/view.php?id=13122.

Nexus 6
Stock android 6.0.1