What to do if you see a message about a "invalid auth code", "access to resource (...) denied", or "500 Error"

The cause:

All of these messages indicate that something's gone wrong with the connection between Pleo and Datev.

The solution:

Disconnect and reconnect to Datev. Before you do so, it can be a good idea to take a screenshot of all the accounts you've selected on the Setup page in Pleo. That will save you a bit of time when you reconnect and have to fill those fields out again.