A red box error means Notes has crashed. There are literally dozens of causes for a red box crash (A search for “red box” on the Lotus support site will return about 40 hits).
In order to fix it you have to first determine what Notes was trying to do that caused the crash.
If it is a custom app, ask the developer to help you. She may have to code around it.
Search the Notes support site to see if there is a patch or workaround.