It looks like your Mendix Connection pool is configured to open 150 connections, but your PostgreSql server is probably configured to accept less connections. So it seems like you have conflicting settings.
Try increasing MaxConnections=150 in your postgresql.conf file.
However I doubt it needs to be set to 150, but the important things is that these settings need to either match, or your Postgres DB needs to accept more connections (MaxConnections) than the value set in your connection pool (ConnectionPoolingMaxActive).
Perhaps you actually need to decrease your ConnectionPoolingMaxActive to be more inline with the database's MaxConnections.
NB. You will need to restart your database if you change MaxConnections!
It might also be worth investigating if your connections are closing properly in which case you could do some investigation:
Run this SQL to see postgresql max connections allowed:
Take a look at exactly who/what/when/where is holding open your connections:
SELECT * FROM pg_stat_activity;
The number of connections currently used is:
SELECT COUNT(*) from pg_stat_activity;