I have been getting complaints about Lazy Approval not working. So I submitted forms on the same list from desktop and mobile, and approved them via email from desktop and mobile. Each time on several tests, they completed. Finally today I saw one fail as I was testing with other users.

I do not see any issues with permissions. The user even completed one that went through, and then on a 2nd one the approval email was captured by the task, but the workflow "Failed." 

Any ideas as to what could be happening??

NOTE: These outcomes are from the same list, same inputs, same user. 

Error:

Successful: