The Pleo expense has a tag that corresponds to a main project in e-conomic. This article explains (in Danish) that expenses in e-conomic cannot be connected to main projects. So the export will fail for expenses with corresponding tags.
In Pleo, remove the tags connected to any main projects in e-conomic and assign it a tag which is not a main project. The Pleo product team is working on an update that will prevent importing tags connected to main projects in e-conomic. This should prevent this error from occurring in the future.
In e-conomic, you can view your main projects by clicking on:
Main projects (Vis hovedprojekter)