no access

I'm studying Modx revolution for a few days;
but today I cant use the manager anymore:
I am logged in as administrator but I have no acces:
when I click right on the templates a popup gives 'undefinded',
all tabs are blank,
the logout popup-screen has no text in it, but I can log out; the login screen is normal

etc...

the website still works; and when I look to the MODX database everything is still there

In the error log:
[2011-11-28 14:41:40] (ERROR @ C:\wamp\www\vierwindenrev\core\model\modx\modlexicon.class.php : 196) PHP warning: array_merge() [<a href='function.array-merge'>function.array-merge</a>]: Argument #2 is not an array

I changed
line 196 from
$entries = array_merge($englishEntries,$entries);
to
$entries = array_merge($englishEntries,(array)$entries);

and now it's resolved.

I don't know the reason that it was suddenly wrong...and how to correct it appropriate but meanwhile I can resume my work

Same here, all of a sudden there was that error.

I just went to the cache folder and removed the lexicon map and some other.

Now it's ok again..