Hi pupbowen,
Sorry to hear a problem has started to occur when trying to split payees. Is there a particular error message that is accompanied in the pop-up screen? If this is a new problem, potentially it could be related to a browser issue which is generating unusual errors or results. I would encourage you to click here to see the browser troubleshoot steps, which includes trying different browsers. Overall still reply back to let me know if there is a certain error message appearing.
Thanks,
-Steven