When creating a request as portal user, I do see that there is insufficient previlage error displayed.
A trigger is fired in the back that send email to the respective owners about this new request. I do see that the request being created.
If I remove all the default users so that the record defaults to the partner user then they can create the record.
Any idea why?
This is because the Permissions for the Org are are not the same as the Permissions within the Partner Portal or Community side of things and your sharing model hasn't created the proper relationships for them to share the same records. You've not provided enough details about how your sharing is set up and what kind of request is being made along with any other relationships involved for me to give a more specific answer than this.
It essentially comes to down to the process on the portal or community side of things being different than the org side of things and not being able to mix the two with the OWD, FLS, sharing groups and permission sets on the objects involved as they're currently configured. I suggest you look at the following resource documents: Security Implementation Guide, Security Workbook and Getting Started With Communities (some of the latter will apply to portals if that's what you're using).
