There is currently a problem with the Xero API connection between Accelo and Xero. An example of the error you'll see if you try and reconnect to Xero is contained below:
From the Xero support team:
We are aware that currently there is an issue affecting connection with all Xero integrations.
It is being urgently investigated and we'll update you as soon as possible.
We'll update this blog post when we have further information from Xero.
Update: the Xero team have ironed out their kinks, and everything looks good now. If you're stuck, make sure you disconnect and reconnect.