@CyanPrime not sure how that's fixed it as your route inside theme.less doesn't actually point to the vanilla template. For that file to point at vanilla, you'd be looking at ../../../nodebb-theme-vanilla, but if it works then, so be it.
Happened on the MangoLassi community. Appears to have happened in someone's chat session. The result didn't bring down the forum, but did break some things like quotes and replies and we could no longer find the end of threads. But mostly the forum was intact. A restart fixed everything, and we've never seen this bug before. But didn't want to fail to report it, either.
We are on NodeBB 1.10.2, all plugins fully up to date (and very few in general), and NodeJS 10.8.0.
2018-11-02T15:50:41.288Z  - error: /user/donahue/chats/2255
at nextTask (/opt/mangolassi-1.0/node_modules/async/dist/async.js:5324:14)
at next (/opt/mangolassi-1.0/node_modules/async/dist/async.js:5331:9)
at iteratorCallback (/opt/mangolassi-1.0/node_modules/async/dist/async.js:1064:13)
at Immediate._onImmediate (/opt/mangolassi-1.0/node_modules/async/dist/async.js:2799:21)
at runCallback (timers.js:694:11)
at tryOnImmediate (timers.js:664:5)
at processImmediate (timers.js:646:5)