There are several reasons why you may encounter "disk space is nearly full" alerts even though cloud storage is unlimited.

Larger Than Cache-Sized Copies to the CacheDrive

For most users, their Internet upload speed is not as fast as the local area network speed, so copying files to the CacheDrive is much faster than the upload from the CacheDrive to the cloud. If the copy size is larger than the available cache space, the upload to cloud may not be able to stay caught up with the local data transfer.

To resolve this issue, break up the local copy into batches and allow time for upload to cloud before copying the next batch. Once files have been uploaded to the cloud, they can be removed from the cache automatically when more free cache space is required.

Unsynced Relicate Jobs

Replicate shares sync with the cloud only at scheduled times or during a manual sync.  Data in replicate shares cannot be removed from the cache until it has been synced with the cloud.

To resolve this issue, manually sync the replicate share to the cloud.  Adjusting the sync schedule for the share can help prevent such issues from happening again.