Don't have an answer for you, but perhaps some explanation based on educated speculation; to change the homepoint, the RC would have to send the new one to the AC, and have it updated in the AC's system. I expect that the way this all works is there is an SDK interface for setting the HP, and it receives a status response from the AC about success/failure.
Litchi is simply getting a failure code response from the AC when it tries to change the HP. First, does the P3S support changing the HP?
If it does, what is the status and history of the firmware? Is it updated to the latest? How long ago was the last time you updated, and does this correlate at all timewise with when this started?
The most likely cause is an incompatibility between the version of Litchi you are running, and the firmware version in the P3S -- that is, one or the other was updated at some point, and are no longer compatible with the other -- i.e. the interfaces for changing the HP in the DJI SDK changed in some way that requires the corresponding app to be updated to use the SDK that way.
A lot of software engineering mumbo jumbo. Bottom line is this: Determine what firmware version is in your P3S, the version of Litchi you are using, then contact Litchi support with this information and the problem. They'll likely be able to tell you right away if this is a known issue, and if there is a app/firmware combination that works.