Well, it's not one specific DAT or one specific file....
I noticed this when I loaded an (updated) TOSEC WIP DAT, directly after upgrading to newly released version of CMP. I went straight to 'scan' (since settings didn't need to be changed) with check/fix everything enabled.
CMP run through all the RARs and renamed any accordingly (no problem), then started renaming files inside the RAR or removed files inside the RAR and thats when it had problems:
"Can't run C:\Program Files\WinRAR\WinRAR.exe!
Please check Settings/Compressor settings"
Same error when rebuilding from a folder with matching files.
Same error when using an existing DAT (i.e working for a year or more already). Have tested a number of DATs, old and new and the problem is consistent.
Interestingly, if you accept the error the file is renamed/rebuilt successfully anyway you just get an error for EVERY file!