This most likely is a mismatch of targets between the C# ASP.NET program and the COBOL .DLL. What is the target of the ASP.NET app? If it is x64 or anyCPU and run on a 64-bit computer then the COBOL .dll must also be compiled as x64.
If the ASP.NET app is targeting x86 or anyCPU and run on a 32-bit computer then the COBOL .dll must be x86.
Any .dlls that this COBOL .dll calls including the run-time cblrtsm.dll must also be the correct bitism.