calling waitForRender() in postProcessRenderedPhotos generates an error ("An unknown error occurred. This photo was not rendered") when called in a "Publish" setting. The same code works just fine in "Export" setting. The same code used to run just fine in the past for several years, this issue is new, a regression.
 
I also was able to reproduce the issue using the creatorfilter.lrdevplugin sample plug-in that is supplied with the SDK.
 
 
To reproduce:
1. Add the plugin in "Plugin Manager"
2. Add the plugin (named "Publish Bug Demo") to an export and export - all should be well
3. Add the plugin to a publish and publish - "An unknown error occurred. This photo was not rendered" is presented for each image
 
Here's the plugin code (2 files):
 
 
 
--[[----------------------------------------------------------------------------
DemoFilterProvider.lua
------------------------------------------------------------------------------]]
-- Lightroom SDK
local LrLogger = import 'LrLogger'
local myLogger = LrLogger( 'libraryLogger' )
myLogger:enable( "print" ) -- or "logfile"
--============================================================================--
local DemoFilterProvider = {}
function DemoFilterProvider.sectionForFilterInDialog(f, p)
	return {
		title = 'Demo Plugin',
		f:row {
			place = "overlapping",
			f:static_text {
						width = leftSideWidth,
						alignment = "center",
						size = 24,
						title = "Demo Plugin"
					}
		}
	}
end
-------------------------------------------------------------------------------
function DemoFilterProvider.postProcessRenderedPhotos( functionContext, filterContext )
	local renditionOptions = {
		plugin = _PLUGIN,
		renditionsToSatisfy = filterContext.renditionsToSatisfy,
		filterSettings = function( renditionToSatisfy, exportSettings )
		end,
	}
			
	
	for sourceRendition, renditionToSatisfy in filterContext:renditions(renditionOptions) do
		local success, pathOrMessage = sourceRendition:waitForRender()
		if success then
-- 		perform some magic
		end
	end
end
function DemoFilterProvider.shouldRenderPhoto( exportSettings, photo )
	return true
end
-------------------------------------------------------------------------------
return DemoFilterProvider
--[[----------------------------------------------------------------------------
Info.lua
--------------------------------------------------------------------------------
------------------------------------------------------------------------------]]
return {
	LrSdkVersion = 3.0,
	
	LrSdkMinimumVersion = 2.0,
	LrPluginName = "Publish Bug Demo",
	
	LrToolkitIdentifier = 'com.yourcompany.yourproduct.lrpi',
	
	LrExportFilterProvider = {
		title = "Publish Bug Demo",
		file = 'DemoFilterProvider.lua',
	},
	
	LrPluginInfoUrl = "http://www.yourcompany.com",
	VERSION = { major=1, minor=2, revision=3, build=4, },
}