When you first connect Pleo to Exact Online, you might already have Exact Online open in a different tab in the same browser. This can cause an issue with the integration setup.
In the Accounting Settings page in Pleo, disconnect from Exact Online:
Revoke access to Pleo from Exact Online Security Center
UserName → My Exact Online → Security Centre → Pleo → Revoke
Log out of your Exact Online account.