I traced the exception to
bsh.Reflect:82 with the line contents:
"return invokeMethod( method, object, args );"
As I'm running a SVN trunk I just added a method.setAccessible(true) and the problem is fixed for me.
Of-course it depends on developers in charge if this should be added to invokeMethod itself or callers should obey a contract of invoking only accessible methods and thus seting the accessibility before calling.
Ticket has been migrated to github.
Please follow up on this over here: https://github.com/beanshell/beanshell/issues/439