Free Space on device, Free space on Icloud+ but Error "not enough space"

Hi all.

I have an Iphone X. with iOS 15.5

I purchased Icloud+ 50GB to have enough space to backup my photos and docs.

Now the device reports 44.7 GB out of 64GB , while iCloud+ reports 32.3 GB out of 50GB.

Nevertheless, Photo reports that the iCloud space is not sufficient; same for Backup.

Where I'm wrong. Of course I rebooted the device, but no way to solve this issue.

Thank you