500 Internal Server Error when Arr attempts to delete failed items via SABnzbd API

[emptyinthehead]

Describe the bug

When Sonarr/Radarr encounters a failed import, it attempts to clear the item from the download queue by sending a standard SABnzbd API delete command to NZBDav:
[GET] /api?mode=history&name=delete&del_files=1&value=[ID]

Instead of processing the deletion or returning a 200/404, NZBDav crashes internally and returns a 500 error with the following payload:
{"status":false,"error":"An error occurred while saving the entity changes. See the inner exception for details."}

Logs:
Warn|HttpClient|HTTP Error - Res: HTTP/1.1 [GET] http://[NZBDAV_IP]:3000/api?mode=history&name=delete&del_files=1&value=ac7e6ab5-a489-404c-a9ae-68d483f1a3af&archive=1&apikey=(removed)&output=json: 500.InternalServerError (114 bytes)
{"status":false,"error":"An error occurred while saving the entity changes. See the inner exception for details."}

Running NZBDav version 0.6.1 and latest sonarr/radarr in Docker on Ubuntu.

To Reproduce
This seems to happen with every "delete" send to NZBDav, at least in my environment.

Expected behavior
NZBDav should not return a 500 error, it should return 200 or 404.

Screenshots

Additional context
If this is not wide-spread issue, let me know if there is something wrong in my environment I should be looking at.

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NOTE:
NZBDAV is intended for use with legally obtained content only. The project maintainers do not condone piracy and will not provide support for users suspected of engaging in copyright infringement. Issues related to or suspected of piracy will be closed.

[emptyinthehead]

Further log analysis reveals a secondary issue that is triggering the 500 error. When an NZB fails due to missing articles, NZBDav correctly drops the internal stream/symlink, but it appears to report the item's status as Completed in the SABnzbd API history payload instead of Failed. Because it reports as Completed, Sonarr attempts to import a non-existent folder, fails, and then issues the delete command that is not trapped properly (deleting a non-existant file) and throws a 500 error. If NZBDav flagged the history item as Failed in the API, Sonarr would handle the failure natively without attempting an import.
I originally thought this happened on every deleted but it clearly does not. This only happens in the case that NZBDav has already deleted the file while Sonarr thought it was Completed. Many other deletes succeed if the file still exists.