Fatal Error: Warning: Invalid argument supplied for foreach()

I am building a site in Revo (2.1/Apache/MySQL5.0.92-community/PHP5.2.17) and was mostly finished when I started getting this error for the front end (below) and a blank screen in the manager. Can anyone tell me what is going on?

Warning: Invalid argument supplied for foreach() in /home/xxx/public_html/core/model/modx/modcontext.class.php on line 49

Warning: array_merge() [function.array-merge]: Argument #2 is not an array in /home/xxx/public_html/core/model/modx/modx.class.php on line 2752

Fatal error: Class ’modUser_’ not found in /home/xxx/public_html/core/xpdo/xpdo.class.php on line 602

What ever you are doing, the value being sent needs to be an array...

As we have almost no relevant information, helping is going to be difficult at best.

Couple of suggestions:

Reinstall MODX over top of itself with a fresh download to make sure all the files and components make it to the server
Re-run setup and perform and upgrade.

Also clear the cache.

We would have to know what you were actually doing, if it is a home brewed Snippet causing this. If it is regular test, you probably have some files not on the server.