First, you need to look at your application server logs to see more detailed information. However, since it says it's a SQL Exception and mentions a Timestamp error, I'll make a few guesses here.
Make sure your using the correct JDBC library for your version of PostGres. jForum uses prepared statements to access the DB. It's the JDBC driver's responsibility to convert a Java Date object to the correct SQL format for your DB server. Date handling is one place that many servers are making syntax changes to become more SQL standards compliant. Old JDBC drivers may not match the new server standards (and vice versa).
It could also be that there is a version specific problem with the table creation scripts (column type defined wrong and the like). Sometimes this can be related to Database/Table default settings that need to be overridden at a column level. (e.g., defining that a timestamp field should use the server Timezone.)
It might also be that one or more SQL queries are not correct for your DB version. The log file should give more indication about where the error is occuring. This can be used to track it down to the query defined in the postgres.sql or generic.sql config files. [originally posted on jforum.net by monroe]
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