Hi @G99 ,
Make sure external users have appropriate permissions on the list item or current list.
If the user still receiving an error message, please remove them from the site using the following steps:
1)Browse to the site and edit the URL by adding the following string to the end of it:
For example, the full URL will resemble the following:
2)Select the person from the list, and then on the Actions menu, select Delete Users from Site Collection.
3)Grant the external user permissions back to the list or list items.
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