When in the Schema Browser, I am not able to see other schemas. When in Treeview, the Other Schemas node is not there. When in Tab or Dropdown view, the other schema are not listed in the field you select the schema from.
There seems to be a filter set that is causing this problem.
Rename the folders that have the name of the databases you are having the problem with in the User Files folder. You can rename the folder by just adding OLD to the end of the folder name. Make sure Toad is closed when renaming these. Here are the full paths to where you can find these folders that have the same name as your database.
C:\Documents and Settings\<User>\Application Data\Quest Software\Toad for Oracle\x.x\User Files for XP
C:\Users\<User>\AppData\Roaming\Quest Software\Toad for Oracle\x.x\User Files for W7
(x.x is the version of Toad that you are using)