As .NET Framework 1.1 is an entirely different codebase from .NET Framework 2.0/3.0/3.5, it could be any number of issues. You may be using deprecated features in 1.1 that are not available in 2.0/3.0/3.5.
If you provide details of the error, readers may be able to assist you. Information needed but missing: Project type, programming language used. Code showing the statement causing the error and a full quote of the error message.