I have my add-ons folder located in iCloud documents. I am not able to install Plugins (e.g. Stacks Plugin or RapidCart), Stacks add-ons itself works fine. I am always getting an error message, console output:
I’ve noticed this “.Update Checker” folder is now in the addons area. I’m afraid I’m totally ignorant of its purposes. I don’t know who is creating it or using it. Well, OK, … I know that it’s definitely not me.
I had a poke around in it just now and it seems to have some of my plugin updates in there – so that would imply that this is part of the RapidWeaver plugin updater.
As for bug reporting (where the goal is to highlight bugs, make them repeatable – i.e. make the worse) I’d say zip up your addons folder and send to RMS with that error message. Maybe if they use it as-is the bug would repeat for them.
As for solving the problem (where the goal is just to eliminate the bug) I’d say start a fresh Addons folder from scratch. Don’t let RW copy of any addons. Add them yourself. Then launch RW, tell it about the new folder, restart. I suspect all will be well.