"Any dynamic maps with the Relief Map layer have the caption "API Key Required" on them since yesterday. (see example below - please change the map layer to the Relief Map layer in order to see the caption)."
The Relief map layers from Thunderforest.com now require an API key to use their map tiles.
Thunderforest (opencyclemap) has recently changed their licensing policy that requires all users of their data to provide an API key.
The official information on how to gain an API key is here: http://www.thunderforest.com/blog/apikeys-now-available/
And how to implement is here: http://www.thunderforest.com/docs/apikeys/
We'll need to look into if Wikivoyage can obtain an API key that would work for all Wikvoyage sites, or if there are limiting license policies that we need to be aware of.