please post your query in this place SAP ERP HCM Employee Self-Service
also please can you check
1. ST22 dump if any
2. goto SAP ECC inbox try to open the workflow , if it is working then the problem is in the portal
3. is it a new implementation ? or exisiting issue?
4. are you getting this in all the client like dev, Qa & prod
5. was there any changes or patches that got recently implemented?
6. is only one user facing this issue or all?
7. what did you tried different machine or IE browser version