I have this exact problem .. :(
I've temporarily solved it by following Zarcon's howto .. I think it's some problem in the caching system :S
#########
I used MyPhpAdmin (in cPanel) to go into my database, then opened the sys_options table
I clicked the SQL Tab and ran the following query:
SELECT * FROM `sys_options` WHERE Name LIKE '%cache%';
I edited the following fields below and removed the 'On' from the value and left it blank (** Note: This will disable all caching for now)
sys_template_cache_enable
sys_template_cache_css_enable
sys_template_cache_js_enable
sys_db_cache_enable
sys_pb_cache_enable
I then cleared my /cache (except for .htaccess) and /cache_public directories. Reloaded my browser and everything worked again. Im still in the process of making sure everything on the site works, but I would say that I am at least 95% done.
#########
Please Boonex .. solved this problem.. so that it works even with caching ON...