Dear Elena
Go by this way you can get to know the reason for parking;
1) Click 'Display Workflow Log'
2) Click on 'List with Technical Details' on top menu icons
3) Expand the nodes and click on the line marked as 'Post Parked document' with error logo
4) After clicking, below you get screen with many tabs. Click the tab 'Container'
5) Scroll down to get to know the reason, as shown below;
Hope this helps.
Thanks
Syed Zia Abbas