After some more investigations I've retrieved the full error message.
Here is the interesting part :
org.postgresql.util.PSQLException: ERROR: function SUBSTR does not exist (Hint: use SUBSTRING instead)
We are using Jasper in Amazon and our data source is stored in a Redshift DB.
Recomendations for Redshift access is to use PostgreSql JDBC driver.
In Jasper Mid function is mapped to SUBSTR PostgreSql function, when in Redshift SUBSTR doesn't exist and should be repaced by SUBSTRING, exactly what I got in error message.
Now I will have to look for the function template file dedicated to Postgresql on Jasper Server and change function definition.
I will try to comment this ticket if it works.