I 'think' this might help
In Designer... Application Properties / Errors and Timeouts / [Check] Display XPage runtime error page.
might give you a little more to go on...
I think there might be an error with the latest build of the extension library... or something... not sure... I have been getting the following error when I try to do anything with the extension library...
Cannot find the library com.ibm.xsp.extlib.library, required by the application /XPagesExt.nsf.