Using MEW 9.10, I have set Beyond Compare (Version 2.0.3, Build 183) as my external diff program.
I have configured my Version Control System to use CVS, but the problem would probably appear with any version control system.
Selecting VCS -> Visual Compare Current File, results in a call to CvsView() in the Cvs.s. This macro checks out a copy of the current file and writes it to a temporary file. The macro routine Fc() is then called, which launches the diff program asynchronously on the current file and the temporary file, but, upon returning to CvsView(), the temporary file is deleted while the external diff program is comparing the files.
With Beyond Compare, this results in a ‘blank’ window, as the temporary file was deleted.