Your PO may be stuck in workflow with error status.
Try with following navigation path :
Login - SYSADMIN > Resp : System Administrator > Workflow > Administrator Workflow > Status Monitor >
Type Internal Name = POAPPRV > User key = (your PO number) > Go
If PO is in error then retry to do the selected activity and restart it with submit button.
Do you know what can be the cause of the encumbrance error?
Sorry dear I dont have idea about Encumbrance.