After updating UPM (now 2.19.4) whenever I upload an add-on from a .jar file, I get a message "Error" in the background in a red box, and the install box with the spinner never closes. I've tried several different plugins (my own jars and downloaded from the marketplace), and all seem to trigger this behavior:
plugin_error.jpg
There are no errors in the log and if I close the browser window and go back into JIRA the plugin seems to have installed just fine, so this seems to be a completely innocuous error.