Some users encounter this problem, and different work-arounds have been found for various DbVisualizer versions.
The main problem seems to be that when connecting using RDP, RDP changes the Windows visual styles (e.g. Aero is changed to plain) and this triggers a change of the Java Look and Feel for DbVisualizer. For some Java versions on Windows (even recent ones, like Java 6 and 7), the Look and Feel cannot be changed dynamically like this, resulting in an error that DbVisualizer is trying to display in an error dialog but due to the underlying cause, it is unreadable.
The best solution is therefore to use plain visuals with no effects on the Windows PC you connect to using RDP. This page describes some alternative RDP adjustments that may also work:
If this is a too drastic solution for you, you can try using one or more of the following Java system properties as described in How do I increase the memory for Java? (just replace the -Xmx1024m property in that example with one of the below):
These properties have resolved the problem for some users of older combinations of DbVisualizer and Windows/Java, but may not do anything for the most recent versions.