Olga,
I believe your users are experiencing an issue where those logged in with credentials of domain\username are experiencing this issue, but if they logged in with an e-mail address the issue doesn't appear. This was addressed in updated build of the Workflow engine. Once you've applied the updated build your users shouldn't experience this issue again.
Here is a link to the updated build.
https://drive.google.com/file/d/0B-FTUltwvJJARS0xRWZFaXBueE0/edit?usp=sharing