CMS: page unresponsive after clicking Base Document

Reproduce: click on Documents/Configuration/defaultcontent/Base Document
(or at Base Document of another namespace)

makes the page unresponsive, a refresh of the page or logout is only action that works.
Other Documen/Compound Types do work.

Attached part of logfile http://cmstest.onehippo.com/logs/hippo-cms.log

firefox httpheader shows a 500 internal error:

GET /?x=geQHVDTg7Ce64*LOYqKC1NfWkI8Whwgr3bY65aoPxpgNDuEO4YKoKTFn8XOMYVg-YhAGwVtpNna4zKkk4t4JD01q*x78RofHGts0okCe3TGMb4KKyBzf*s2PjozUyksmFpojMk1aBXRcn6zuLjNGE07X6FjbEAVC0eVcZBJbRF8&random=0.09982577372933232 HTTP/1.1
Host: cmstest.onehippo.com
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.6) Gecko/20100628 Ubuntu/10.04 (lucid) Firefox/3.6.6
Accept: text/xml
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 115
Connection: keep-alive
Wicket-Ajax: true
Referer: http://cmstest.onehippo.com/?x=CN8dnOm1j*A1r1gl3QQLGCD03WLgsE2rnvZ0Dwo-FlBP1vx5v3FIqRXQBo9JIZTn
Cookie: hippocms7-layout-sizes=navigator-wrapper%3Aleft=200%2C862&template-type-wrapper%3Aright=250%2C834; JSESSIONID=4FC8067CA56DAE49ED45457B340D58CB; Hippo_scroll_offsetX=0; Hippo_scroll_offsetY=0

HTTP/1.1 500 Internal Server Error
Date: Mon, 12 Jul 2010 13:33:55 GMT
Server: Apache-Coyote/1.1
Expires: Sat, 26 Jul 1997 05:00:00 GMT
Cache-Control: no-cache, must-revalidate
Pragma: no-cache
Content-Type: text/xml;charset=UTF-8
Via: 1.1 cmstest.onehippo.com
Vary: Accept-Encoding
Content-Encoding: gzip
Content-Length: 2549
Connection: close