Hi,
to sum it up:
* The problem was due to commata in page titles (c.f.
http://jira.xwiki.org/browse/XE-1149).
* I had this feature
(
http://extensions.xwiki.org/xwiki/bin/view/Extension/Watchlist+Application#…)
activated, which lead to the problem (but does not directly cause it).
* I could solve it by the workaround of going to my profile page, accessing the objects
and remove all content from the field "Document list" in my
XWiki.WatchListClass.
* Problem will reoccur when adding commata-titled-pages to my watchlist.
Bests,
Moritz