i think it was my fault - I used a different name than "jdbc/Main" in my Configuration.xml as described here. To me it was not clear, that I have to use "jdbc/Main". I thoguth it's just a name for the connection.
I changed the connection name to "jdbc/Main" and it works. If I changed the name back to an other name the HSQLDB is used again.
I think i've got the idea behind it now. "jdbc/Main" indicates the main data-source, any other name can be used for an additional datasorce, right?
Maybe you can write that down more clearly in the documentation. I think it woud be helpful to save some time.
Again, thanks for your fast response!
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