After updating, getting "Could not resolve the subpath" error After updating to 2.6.2973, saving any entry fails, resulting in an Internal Server Error: "Could not resolve the subpath '{slug}'".

I've read the similar errors on here which trace back to Matrix Asset objects and dynamically-named folders, though I'm not using any dynamically-named folders, and I can't tell where it's pulling {slug} from, and/or what's causes the issue. I've gone through and double-checked and resaved both asset directory and field configurations.

The log again points in the direction of the Matrix Asset, and refers to the folder id "1" -- a Local Source which has both the System Path and URL completely hardcoded. The top of the trace looks like so:

#0 /[directory-path]/craft/app/fieldtypes/AssetsFieldType.php(707): Craft\AssetsFieldType-&gt;_resolveSourcePathToFolderId('1', '{slug}', true)
#1 /[directory-path]/craft/app/fieldtypes/AssetsFieldType.php(327): Craft\AssetsFieldType-&gt;_determineUploadFolderId(Object(Craft\Model))
#2 /[directory-path]/craft/app/services/ElementsService.php(1618): Craft\AssetsFieldType-&gt;onAfterElementSave()
#3 /[directory-path]/craft/app/services/MatrixService.php(778): Craft\ElementsService-&gt;saveElement(Object(Craft\MatrixBlockModel), false)
#4 /[directory-path]/craft/app/services/MatrixService.php(882): Craft\MatrixService-&gt;saveBlock(Object(Craft\MatrixBlockModel), false)
#5 /[directory-path]/craft/app/fieldtypes/MatrixFieldType.php(533): Craft\MatrixService-&gt;saveField(Object(Craft\MatrixFieldType))


This error stalls all the client's content entry, so I hoping for any additional insights or tips. Thanks so much.
If you're setting an dynamic upload folder location for an Assets field inside of a Matrix field, you need to preface things like {slug} with {owner.slug}.

See the note about dynamic subfolders on an Assets field inside of Matrix here: https://craftcms.com/docs/assets-fields#dynamic-subfolder-paths