iCloud Backup option greyed out on iPhone 6S Plus and iPad Pro

I sync a lot of files on iBooks (PDFs) via iCloud and this morning I went to look for one and it had gone from my phone. My investigations took me to look at iCloud back-up and that was greyed out. Lots more investigating and I found that my iPad Air had also lost all the PDFs in iBooks and also back-up greyed out. However my new iPad Pro was not greyed out. The last couple of posts have given me the probable cause. My iPhone and iPad Air have work email (Blackberry) on them and the iPad Pro does not. Last week they did a major upgrade and looking at the last back-ups that when it happened. I suspect this was intended and added as an additional security measure. Major headache for me unless it can be resolved. I will investigate next week.

Make sure both devices are connected to a Wi-Fi network. iCloud requires Wi-Fi to backup your data.

If it's not a Wi-Fi issue, you may need more iCloud storage.

Tap Settings > iCloud > Storage

At the bottom of the screen you should see how much available storage there is. Less than 1GB might be preventing your devices from backing up.

This article might also help:

Get help backing up your device in iCloud - Apple Support

Hello, same problem here. What was the fix? Thanks.

How did you deal with it? I also have my work emails on this phone and am facing the same issue. Is there no way out?