You should try and install the XPages Log Reader application on your server. This harmless app is on OpenNTF and would give you good access to error messages including the full stack trace. To me this is a must have application on every Domino XPages server.
Again - you're not giving any code so it's very hard to really advice you. But from the message you getting s Selection object when you're expecting a Vector. The objects are not the same. That's the problem.