InvalidOperationException can have two causes - it either means that the error is in your program (e.g. it is not possible to call the GetFile method before you are connected and logged into the server via Connect and Login methods; or another example would be if you do not handle a previous exception correctly and then call another Ftp method, then the InvalidOperationException would most probably be thrown as well - the solution is usually to fix the recovery from the previously badly treated exception.)
Another possibility is that there is a problem in the Rebex FTP component.
If you are unable to identify the problem in your program based on the actual error message, we would be able to look into it and help you. However, we would need to see a log of communication then. In this case, either post your log here, or send it to firstname.lastname@example.org. Based on this we will be able to tell you more.