Hello Mail,
you should check that the system aliases for eRec are defined in the portal system configuration to target the correct system when eRec is called in the portal. Furthermore correct data is necessary for calling the eRec application (e.g. user needsa candidate).
To narrow your error could you provide additional information? Are there any errors in the application log (TCode SLG1), any runtime errors (TCode ST22) or system errors (TCode SM21). It would also be helpful to know if you are using the "old" ESS or the Web Dynpro for ABAP ESS.
If you want to investigate the issue on your own. I suggest to try to call the startpage for the user directly via link. That way it is easy to find out if it is an eRec issue or a Portal setup issue.
Kind Regards
Roman
PS: as a general side note. If you are using the new WD4A ESS with the launchpad I suggest to directly link the different eRec applications in a "Recruitment" section instead of linking the startpage. This usually makes a better user experience.