I have this script that uses ifcmp90 to compile all forms in one directory. I sometimes need it when I change a library, but lately it has been failing me with an ORA-12538: No such protocol adapter. I had been fiddling around with my oracle homes and apparently I broke this.
It turned out that the error occurs when you have your oracle_home environment variable not set correctly. It is usually set to the home of the database but for ifcmp90 i needed to set it to the devsuite home.
This solved it