Hi there,
I also had the same problem. Nothing in the documentation, nothing on the Internet. I've created an ftp-repository as well as a http-repository. Both had the same errors.
Finally I found a solution:
At the epo-server create a Scheduled Task and choose for an ePolicy Orchestrator Agent Update. When you edit the task settings of this task, choose "this task updates only the following components" and select "Extra DAT", "DAT", "Engine" and (at the part of Patches and Service Packs) "ePo Agent 3.5.0". You also can schedule this task to run every day at 6.00 AM (e.g.).
I think this issue only occurs when you have selected "No" at the option "Enabled global updating" (at your ePolicy-server, above the level "Directory").
You can also change this setting at a higher level (but then you'll have to choose "Yes" at "Enable global updating", that's up to you). Go to "Settings" in your ePolicy-application, enable "global updating", Select "Extra DAT", "DAT", "Engine" and (at the part of Patches and Service Packs) "ePo Agent 3.5.0". Last part is choosing an update interval.
So, this issue has something to do with updating your ePO Agent-client. You can do this without global updating and using a scheduled task or do this with global updating and using an update interval.
Hopefully this helps you to solve your problem, it certainly did solve mine