What's causing the issue?

The most common cause for this issue is copying over Mac files from a Windows device to the CacheDrive.

For Mac compatibility, Windows uses alternate data streams (ADS) to hold Mac system properties lists.  See the following article for more information about alternate data streams:

Macintosh Supported Stream Names

CacheDrives are capable of handling AFP_Record alternate data streams of up to 16000 bytes in length.  If the ADS exceeds this length, you may encounter an out of space error:

How do I determine that this is the problem?

To determine the length of a file's AFP_Record ADS, use the following command in Powershell:

get-item $filename -stream *

Look for the AFP_Record stream and the length beneath it:

How do I fix it?

One method for removing the ADS would be the following Powershell command:

remove-item -path $filename -stream AFP_Resource

You should then be able to copy the file over normally.