I've been dealing with the same issue and have just managed to fix it after dealing with both Apple and WhatsApp support.
So, combining the info from both groups, Apple and WhatsApp, here are the steps that you should take. Hopefully you won’t have to do them all and it will work after step 1:
1. Check that you have enough iCloud storage space. Apparently, you need 2.05 times the amount of iCloud storage space than the size of your backup so in your case you would need almost 4.8GB of free iCloud space (2.33GB x 2.05 = 4.78GB). You have 5.1GB free so this may not be the problem although it may be worth upgrading and then trying a new backup or leaving it to see if your backup finishes.
2. Sign out of iCloud on your iPhone, turn the iPhone off and back on, sign back into iCloud and try another backup.
3. If still not successful, reset network settings, then go into iCloud settings and then turn off iCloud Drive, then go to Manage Storage and delete your WhatsApp backup (if you’re using iCloud Backup you’ll need to delete the WhatsApp data there also). If when you delete the data your iCloud usage does not change (i.e. go down) you will need to contact Apple support and get them to do a rebuild of your iCloud storage. Once this is done you will most likely have to repeat the deleting of your WhatsApp backup data. Make sure WhatsApp is closed and then repeat step 2 above.
This should resolve the issues you are having with your WhatsApp backup. Hope it helps!