Welcome to our Community 
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when you say you cannot access the files on the other device, do you mean these files are not visible or are present but not useable (e.g. for opening, renaming, etc) ?
Please ensure that your cloud is fully synced. We had several reports specifically to iCloud (e.g. this or that one) that some directories were not uploaded to the cloud.
You can test this by unlocking your vault and place a file in a not synced directory inside the vault. iCloud should provide a view on the most recently synced files, therefore you can check if the encrypted version got uploaded. Additionally check if the encrypted file is present on the other devices.