Happily running Cura 4.2.1 on a Mac (OS X 10.12.6). Using Octoprint on a Pi for a year or so and learned about the OctoPrintPlugin the other day. It does not show up in my Marketplace as I searched and came across this thread. Per a post above, downloaded "OctoPrintPlugin-v6.0.0-2019-07-18T12_32_22Z.curapackage" and dropped in onto Cura. Got the popup that it would be installed on restart. Restarted. Looked into Cura.log and found the following entry:
Deleted all of what had not been installed (.../Library/Application Support/cura/4.4/plugins/OctoPrintPlugin/*).
Downloaded the latest version of the repository into .../plugins/OctoPrintPlugin/, changed subdirectory name from "Cura-OctoPrintPlugin-3.5" to "OctoPrintPlugin". NOW under printers, I see the Connect OctoPrint button (!). However, my Octoprint instance is not discovered. So I enter the IP address. The settings dialog appears briefly and disappears. After tinkering a bit, I get the dialog to stay open long enough to request an API key, which Octoprint grants and Cura crashes. Enter details in the crash reporter which does not close when I click Send. Eventually I close the crash reporter. Re-open Cura and am told that my profile is corrupt (I only have one printer) and would I like to reset? I DO NOT say yes as I do not want to re-tune my Cura settings yet again (screen cap below).
Cura continues to function, as near as I can tell. I re-loaded a stl previously sliced and it told me 1hr 40min to print which was over twice what it has said last time I had printed it. Loaded a new stl (8mm x 20mm solid cylinder) and got a print time of 10min.
Things slice and print but the Cura crashes and it's clear I am going to do what nobody wants to do and start from scratch. But this time maybe I'll RTFM and use material profiles correctly.
*sigh*