Have you tried the other way around:
1. Select folder containing assets.
2. Click on Timeline from left hand side toggle.
3. Click on Start workflow and select the dam Update Asset workflow.
Check the workflow instance from crxde: /var/workflow/instances/server0
If error messages do not provide much info, then put debug log on com.adobe.granite.workflow.core and com.day.cq.workflow and replicate the issue.