'javaws' received an X Window System error

[helio]

IcedTea JDK 6.1.12.6 is my Java vm. When I run a JNLP application from Firefox using javaws, I got the following error:

The program 'javaws' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadMatch (invalid parameter attributes)'.
(Details: serial 3946 error_code 8 request_code 139 minor_code 4)
(Note to programmers: normally, X errors are reported asynchronously;
that is, you will receive the error a while after causing it.
To debug your program, run it with the --sync command line
option to change this behavior. You can then get a meaningful
backtrace from your debugger if you break on the gdk_x_error() function.)

However, I only have the above error when running Firefox and javaws as root. When running javaws as user, I don't have the above error message.

Any suggestions are appreciated.