[dshorrosh]

I just upgraded from Krypton / Mimic to Nexus / Mimic-LR, and for some reason I cannot attach thumbnails to *some* ripped concert DVDs.

Thumbnails are stored in the same folder as the ISOs and referenced in the matching .RTF file with concert info, and this process works for the rest of the ripped concerts.  I try to manually assign art via right-click 'info' but it doesn't 'stick', and the other three concerts with this problem don't even show the right-click 'info' options.  They're all ISOs with matching named RTF, NFO and JPG files.

It's a head-scratcher.

[randallspicher]

Mimic itself does not do any of the library scanning.  The Kodi core is responsible for that.  The skin only displays whatever is returned by Kodi for the various artwork types.

I'd note that you are jumping quite a few versions, and I believe the nfo structure changed between v17 and v18, so you may need to revisit and update your nfo structure.

https://kodi.wiki/view/NFO_files/Movies

And are you referring to videos that have been fully scanned into the library?  Or videos you are viewing only in filesystem mode?  Most artwork types are only available from kodi for videos that have been fully scanned into the library DB.  In filesystem mode kodi will try to extract a preview thumbnail screenshot from the video on the fly, but that does not work for ISO files.  And it would not have a record in the DB to store your artwork choices unless the video is actualy in the DB.  (when in a filesystem view, kodi will lookup and display the metadata and artwork if the video was actually scanned in and has a DB record, but anything not scanned in won't have data available in the DB) 

How is your artwork named?  I'd note that some of the artwork behavior changed in kodi at some point in the past few years.  Are you referring to poster, landscape, or fanart?  Mimic currently allows selecting between various artwork preferences in most views, but again, that mostly only applies to videos that have been scanned into the library and populated all the artwork types.

https://kodi.wiki/view/Artwork_types

I've never heard of using RTF for metadata, and a search of the kodi wiki says there a no references to RTF.  So I have no idea what you are referring to on that.

Also, with that large of a version jump, if you had let kodi "upgrade" the DB, it might not have been totally successful.  It may be necessary to delete the DB and rescan everything from scratch.