The following is the description of the Oracle error you are receiving:
"When ORA-00904 occurs, you must enter a valid column name as it is either missing or the one entered is invalid. The "invalid identifier" most common happens when you are referencing an invalid alias in a select statement. " (http://www.dba-oracle.com/t_ora_00904_string_invalid_identifier.htm)
Perhaps one of your custom forms refers to a field that has changed name or been removed?
At what point do you get this error ? During the upgrade ? When you try to log in for the first time afterwards ? Accessing a particular form ?
You should check the log files from the upgrade - it should tell you if something didn't work as expected.
Did you have to make any changes to the database as part of the upgrade ? Any field deletions / renames ?
Unfortunately I don't know which log file might help in this, but it may be worth running a sql trace as it may tell you which form is the problem. Get your oracle DBA to help you with this.