Error when adding Skills on a Requisition

We are testing Dynamic Skills within Recruiting, and are able to add Skills when creating a Requisition. If I then try to add a Skill onto an Open requisition, I receive the below error when trying to Save. I can delete Skills that were added previously, but if I try to add the same Skill (or a completely different one) back onto the Requisition, I get the error. I don't think it can be due to an issue with the Skills themselves, as I can add then at the creation stage.

Does anyone now what could be causing this error?