Probably a DLL in your GAC does not have the same symbols which are in your dev bin folder. Overwrite the DLL with your development one and most probably the error will be gone.
“While debugging in Visual Studio, click on Debug > Windows > Modules. The IDE will dock a Modules window, showing all the modules that have been loaded for your project.
Look for your project’s DLL, and check the Symbol Status for it.
If it says Symbols Loaded, then you’re golden. If it says something like Cannot find or open the PDB file, right-click on your module, select Load Symbols, and browse to the path of your PDB.
I’ve found that it’s sometimes necessary to
stop the debugger
close the IDE
close the hosting application
nuke the obj and bin folders
restart the IDE
rebuild the project
go through the Modules window again
Once you browse to the location of your PDB file, the Symbol Status should change to Symbols Loaded, and you should now be able to set and catch a breakpoint at your line in code.”
quote from http://geekswithblogs.net/dbutscher/archive/2007/06/26/113472.aspx