Small issue:
  Today I had an issue with some workflow task items, created by a Nintex Workflow which suddenly could not be opened by end users (by assignees that is): error below.

It was working for me and since I have higher permission on the site, after scratching my head a bit, I looked at the item permission : it was fine.
The issue laid with the actual “RelatedContent” item, ie. the item on which the workflow ran when it created the workflow task, the permission for this item were reduced to only the Author of the item and his/her manager, and it was missing some groups like Viewers.

Fix:
Adding the group to item level permission of the RelatedTo item fixed the issue and users could access the task again.

Root cause:

• – some SharePoint groups have been renamed and the workflow which assigned the permission was trying to add the old names, resulting in none being added. The workflow permission action had to be re-done.
• – although the workflow task item should be independent from the source item it is not because the Workflow task item also has an embedded webpart that is -sometimes- used to display columns from the RelatedContent item, therefore if that values cannot be read the whole task item errors.

Conclusion:

– be careful when renaming SharePoint group
– an item error may not be linked to the item itself