[jjd-uk]

I would suggest enabling Debug Logging again then via the Kodi interface go to Music -> Files and navigate to the album folder but do not go into the album folder, with album folder selected open Context menu and select "Scan to library" wait for scan to complete then if still not added to Library close Kodi and provide the link to the new log file on pastebin/paste.ubuntu.

[foo-on-air]

I haven't used kodi UI itself at any point. All was done from the official kodi remote app, like updating the library (there is a command button) or enabling debug logging. In file view the missing albums are visible. Unfortunately via remote app it wasn't possible to scan items into the library because there is no context menu. So I did as jjd-uk suggested. Connected a display to the MC, activated debugging, browsed to the albums in file view and scanned them one by one. The first album was added by right clicking on the artist name (upper folder) which contains a sub folder that has the files in it. The second album was added by scanning directly the sub folder containing all the music files. Please find the log on pastebin

[DaveBlake]

Looks to me that this time the albums were successfully added to the library.

So that leaves the question of why calling update library from the remote app didn't cause that to happen previously, and why from the log it didn't even see the folders let alone try to scan the metadata from the new music files. I really don't know what would cause that.

The library update process checks hash values for every folder as it loops through the folder hierarchy, only drilling downwards if the hash (based on name, timestamp and size) has changed. I guess something in that area failing, perhaps a data corruption, or folder properties not changing as expected. Then again the previous logs would have shown a folder in the tree being skipped, and I didn't spot that (but the log was partial).

[foo-on-air]

Two strange things are happening here. First after adding them manually they still wont show! Second other albums are vanishing. I had one that had already been added to the lib and now it has fallen out of the index. I triggered an update and still it wont get added again! As if there was data corruption occurring! But when accessing the files in folder view they playback fine without errors. Should i erase the whole lib and add the music folder anew?

[foo-on-air]

No further assumptions? I wiped the library clean and added the folder from anew. Also the previously missing albums have been indexed and are visible now. Maybe a confusion with the hashes as you suggested.