If you have access to the K2 Management site, try drilling down into the Users → Task List section. In there you can see exactly who this task was assigned to and it might help clear things up.

Please check the status of task . This could be that other user has already opened task and it’s allocated now.

Hi @bentsui 
Have you resolved this issue? 

Yes. I found that it was related to other settings. 

Thank you @bentsui!

Would you please share with us what they were so I can add them as the solution to this question, to help our other users.

I create the URL to open the form manually. I use the system variable ‘Step - Serial Number’ to create the URL. The number may be incorrect when there are more than one participants. I finally add the participants as a group to solve.

Thank you!