My reMarkable 2 is connected via USB, and I can successfully ssh to it at IP address 10.11.99.1. I have the plugin configured as follows:

(the password is the correct one)
And yet, when I start Calibre, it doesn't see the device. When I run calibre-debug -g, I see this in the output, but no more details:
Error during device detection for <calibre_plugins.remarkable_plugin.RemarkablePlugin object at 0x000001FF7A2DFF70>:
Traceback (most recent call last):
File "calibre\gui2\device.py", line 291, in detect_device
File "calibre_plugins.remarkable_plugin.__init__", line 50, in detect_managed_devices
AttributeError: 'RemarkablePlugin' object has no attribute 'seen_device'