This tends to happen when you are trying tp open a newer .ai file than corel can accept. so you need backwards compatibility on your .ai files.
example: your using corel 11, your client is using illustrator CS2, CS2 is newer than corel 11 so corel will struggle to open the CS2 file. You need your .ai files saved down to a earlier version (exported as a legacy file) something like .ai 8 would do fine.