i had a problem with one of my workflow using tasks (task or flexi task). When a task is submited the users can answer once, then the workflow should move to next step but it didn't work and stayed stucked at pending status. If any user tries to revalidate this task he will have an error "The task is locked by the workflow ...".
I found out that this issue was linked to the email notification attached to my task. There was some errors on my workflows linked to email errors (because i used for tests accounts without emails ...). Then i removed the sending email action to this task and it worked ... It seems that the email error locked my task like if it was a prerequisite to complete it, without email the task is note completed.
I'm using Nintex for a while and i don't remember having a problem like that. My solution in that case was to set email to every accounts (including tests) but i guess there another (clean) way to avoid this kind of problem with Nintex.
Maybe it will helps someone in the future, if anyone have a better explanation, please let a comment on this post .