In DSpace 3.0, we accidentally re-introduced the bug described in
DS-1031, by removing the Harvesting configs.
Essentially, all of the OAI Harvester settings which used to be in the "oai.cfg" file have been removed. This causes the XMLUI's OAI harvester to not work properly.
This was fixed on Dec 6 by this commit:
So, this will be resolved in 3.1. However, if this bug affects you, you can simply add the missing configurations (as shown in the commit linked above) to your config/modules/oai.cfg file.