if you are on a layout based on the Offertes TO and have a portal pointing to the Offerte_Facturen_log_2 TO and have an instance of a field from Offerte_Facturen_log_2 in the portal and have no filtering set up in the relationship OR the portal then the only thing that would prevent you from seeing records in the portal is the data.
Make sure that Offertes::id and Offerte_Facturen_log_2::id_Offertes have values in the records that would actually produce matches to show in the portal.
sometimes people duplicate a portal, change the portal relationship but forget to also change the fields in the portal to reflect the correct ones, maybe is this your case ?
After some checking I found that the source was incorrect.
I thought I checked it a hundred times before asking here, guess I need some more breaks in between doing this, haha