Why does slic3r get an error "302 moved temporarily" when uploading to OctoPrint?
Specific answer: use "octopi.local" rather than "octopi", since that will properly resolve to the correct Octopi IP Address.



More generally, investigating the network traffic on my network (AT&amp;T Fiber Home) revealed these facts regarding the octopi server:


With a browser, the octopi can be accessed via either "octopi:80" or "octopi.local:80".
However, "octopi" actually talks to the network router, which sends a "302" message with the target name "octopi.local".
So, for the browser it seems either name is correct, but in actuality only "octopi.local" is the correct network name for the octopi system.


And these facts regarding the slic3r configuration:


slic3r does not process "302" messages, instead treating them as errors.  This is OK, since a properly configured network won't be generating these.
the slic3r connection test is incomplete.  It reports success because it receives an HTTP response, even though the response is a 302 rather than the version information that it is requesting.


So in this particular case, entering "octopi.local" instead of "octopi" corrected the error.  If you receive a 302 error, you will need to research and verify the exact IP address or host name for the target Octopi system.

A future version of slic3r should probably report an error on the "test" button if it doesn't (a) get a proper 200 return code and (b) return the proper version information as slic3r is requesting.