Let's begin by saying that compression is never recommended. A backup needs to be reliable. Compression introduces to many potential problems.

If you use zip compression (or encryption with AES) you may find that some programs like Windows Explorer or Winrar cannot open your zip files if:

1) You are using Zip64. Not all zip programs understand this format
2) You are using strong AES encryption. Not all zip programs understand this format

Solution 1:
Use Winzip or the Decompressor included with Cobian Backup to unpack your files

Solution 2:
If you want to create STANDARD zip files that can be open by any program then:

a) Set Zip64 to Never in Option-Compression (This will limit the size of your files to 2 GB)
b) In option-Compression check "Use weak encryption". This will override the Strong Encryption setting, but be aware that the PKZip encryption is very weak.