Thank you for your help.
>Did you try to use Derby?
No, I din't. Why do you think Derby is better than postgres? When I migrate to Derby, is there a possibility to (automatically) migrate also the data in the database?
>Can you change javax.sql.ConnectionPoolDataSource to javax.sql.DataSource? Or use XA datasource altogether?
I changed the setting to javax.sql.DataSource, but it had not effect. What do you mean with "use XA datasource altogether"? Where can I configure it?
But I found something what helped: I increased the "Initial and Minimum Pool Size" from 32 (default) to 64. The error vanished, but I don't know if it just depends on how many times per timer event a connection is requested. May be the error will occure again with more connection requests. Do you now what the maximal possible value is for the Initial and Minimum Pool Size? The value of 128 seemed not to be accepted.
[Message sent by forum member 'glassfox' (glassfox)]
http://forums.java.net/jive/thread.jspa?messageID=337641