When sending many files to Multipar I get "Filename or Extention too long" error.
Will you capture the screen-shot of the error message and post here or send by mail ? If it is an error of your OS, I cannot help you, because it is a problem of Microsoft. The maximum number of possible characters depends on OS version.
You may use "Shell Extension (Integrate MultiPar into Shell)" instead of [SendTo] menu, if you select many files often. When many files are selected, Shell Extension DLL uses a temporary file to send filenames, then there is no limit number.