Task cannot be done

First of all, Thanks for your great application.

I have multiple projects.
Other projects are all fines, but tasks in a single project cannot be marked as done.
Whenever I click the mark as done button, task page change to blank task with numbering #0.

I already try in demo, but I cannot reproduce the error.
Even restart vikunja, postgres linked to it not worked.

Is there any possible reason and way to resolve it?

Are there any errors in the logs? Browser or Vikunja server?