I see the same bug. When clicking an affected task, I see the following javascript error in the Chrome console:

compiled.2076.js:17358 Uncaught
Eu {message: "Error in protected function: b.Ub is not a function", stack: "TypeError: b.Ub is not a function↵ at Fz (https…themilk.com/app/_/2076/compiled.2076.js:24652:15)"}
message
:
"Error in protected function: b.Ub is not a function"
stack
:
"TypeError: b.Ub is not a function↵ at Fz (https://www.rememberthemilk.com/app/_/2076/compiled.2076.js:21088:23)↵ at dD.d.OD (https://www.rememberthemilk.com/app/_/2076/compiled.2076.js:24736:11)↵ at https://www.rememberthemilk.com/app/_/2076/compiled.2076.js:62969:7↵ at Array.reduce (native)↵ at Array.reduce.f (https://www.rememberthemilk.com/app/_/2076/compiled.2076.js:625:33)↵ at dDa (https://www.rememberthemilk.com/app/_/2076/compiled.2076.js:62968:14)↵ at k2.d.Wx (https://www.rememberthemilk.com/app/_/2076/compiled.2076.js:62980:33)↵ at jg (https://www.rememberthemilk.com/app/_/2076/compiled.2076.js:2788:20)↵ at dD.d.dispatchEvent (https://www.rememberthemilk.com/app/_/2076/compiled.2076.js:2755:64)↵ at dD.d.T5 (https://www.rememberthemilk.com/app/_/2076/compiled.2076.js:24652:15)"

And voila, it's fixed now, looks like an update was pushed. Thanks RTM team ;)

Yes, just reloaded RTM and the problem is solved.
Set up some Favourites and really liking this new feature.
Shame the problem happened, but with software it is inevitable.
Well done to RTM for a swift resolution and also for continued improvements.