I have configured slic3r's OctoPrint Upload section with the with hostname ("octopi") and port (80) of my OctoPrint server. The Test button indicates a successful connection.
But when I use "Send to Printer" I get an HTTP error 302.
How can I fix this? 
1 Answer
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&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.