We have upgraded to Play 2.7.0 recently and use play2-reactivemongo version 0.16.2 with reactivemongo 0.16.3. We use reactivemongo-shaded-native but also tried
It seems you reached the limit of connections available: have you tried tuning the connection parameters http://reactivemongo.org/releases/0.11/documentation/tutorial/connect-database.html? Atlas usually supports more than 20 connections https://docs.atlas.mongodb.com/connection-limits/
I solved a similar issue with mongo native scala driver setting a pool of 50 connections, a longer wait queue and longer wait time (due latency in connection with Atlas)