In the forum, Carl Daniel reported getting the message "Invalid access code (bad parameter)" when comparing two different versions of a file in Team Foundation Source Control Beta 3.
Unfortunately this is a bug that was discovered and fixed after beta 3 was released. The problem is that large files (on the order of 500K) will crash the diffmerge.exe tool. I'm not certain as to the exact size that begins to trigger the problem.
If you hit this bug, you'll need to work around it by using another diff tool until a newer public release fixes the problem (the upcoming beta 3 refresh will still have this bug). Of course, you could also reduce the size of the file, but that's not often an option.