Unable to complete a task: error message "Your task change couldn't be saved"

[Franko]

Attempting to compete a task from a task list that lists all tasks assigned to me.

When I check the task an error pop-up appears stating "Your task change couldn't be saved".  Looking at the network traffic the response says "Specified task(s) not found"

Next to the task is a link to the document where the task is located.  Viewing the document I do not see the task anywhere.

[Brant Schroeder]

I would suggest raising a request with Atlassian as they will be able to look at the logs and see what the issue is and fix it.  Raise a ticket with Support.