I have an HS105, Firmware 1.0.18 and am having the same issue. Log files for your use attached.
octoprint (1).log (5.5 KB) plugin_tplinksmartplug_debug.log (4.6 KB)
Yep, I think you might be running into a firmware issue potentially with your plug. There have been reports of recent firmware updates breaking the capability of communicating directly to these devices not only in this project but also here and documented here on the github readme. From my understanding if you never connect to cloud control or update the firmware on new devices it may still work. Not sure if you can revert firmware or not.
I uninstalled it and reinstalled the plugin, and today it has started working again 
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I'm an idiot and changed the IP on the Shutdown Printer plugin but didn't do the same on your plugin (TPL). Still working on firmware 1.0.18.
I have a question about the plugin. Is there any way to issue a GCode command as soon as it starts? I’m running octoklipper and for some reason, when I power on the printer, via the TPLink plugin, I need to do a firmware restart via GCode command before it will work. Is there any way to have a wait for a few seconds and then run the command?
No, but there is a feature request for that specific use case.