Cannot delete files_external_onedrive in Nextcloud

[martinlentink]

I have an annoying error with one of my Nextcloud users. They had previously installed the files_external_onedrive app, which worked for a period. Then it stopped working after a Nextcloud update, most likely because it didn’t have an updated version. After this the app was apparently no longer maintained, and the user uninstalled the app. The folder for it however still shows in their folder list and in their settings page for external storage the share is still listed as external storage, with an error saying the reference to files_external_onedrive is unknown (which is obvious, as the app had been uninstalled).
I have looked everywhere in Nextcloud's settings, as well as in the database, but so far have been unable to fix this. Could you tell me how I can get rid of both the entry in the folder list and the entry in the external storages page for this user?

Thanks in advance!