We have an annual archival workflow which copies list items from one sharepoint list to a new sharepoint list. However, if the list items have attachments with filenames containing "#", the workflow fails. I met with a Nintex support engineer who confirmed this is a known bug on the Nintex side, and he suggested to create this Nintex Idea here. From our team's perspective, this is a bug which should be fixed.
Please see attachment for a screenshot of the error.
Reference: Nintex Support Case #00523670