We have implemented a central token store for making multiple Podio API requests from AWS Lambda using the same access tokens. I have been seeing frequent "unauthorized" exceptions returned from Podio. When I use the token from the "unauthorized" request directly in REST Client it works fine. In addition, the rate limit values show that I am not near the limit. Is there a limit to the number of concurrent requests in Podio by account? Thanks for any help.
If Podio returns 403 Unauthorized then access token you've provided is not valid for resource requested. And there is no way that same request for same resource with same access token that got 403 once will get successful response when you run it from another client.
If you want to troubleshoot it well: record/log full https request and response.
Regarding rate limit part of your question: there is different limit for login operation. And there is no general limit on number on concurrent requests by account, but load balancer and DDOS protection might kick in if you go insane.