I’m affraid your MEW 9.1 installation is broken. The error means you’re probably using old MsBsc.dll file which is located in MEW root directory. Could you post DIR listing of your MEW root directory here? You can have other files broken as well.
Correct MsBsc.dll version should be dated 3.12.2003 and 32 kB size. Older version installed with MEW 9.0 is 40 kB sized and from 2000 or 2001.
I’m surprised you didn’t see this error before. It has nothing to do with deleted temporary files. Haven’t you copied some files from old installation by hand later? Also, I believe if you skip the error, BSC should work.