Still not sure what’s causing the problem, but here are the direct links to the pages in question. (People who are having trouble – if you enter these directly, does it work?)
https://www.lesswrong.com/allPosts?after=2018-01-01&before=2019-01-01&limit=20&timeframe=monthly&includeShortform=false&reverse=true
https://www.lesswrong.com/allPosts?after=2018-01-01&before=2019-01-01&limit=100&timeframe=allTime
Those both work.
Still not sure what’s causing the problem, but here are the direct links to the pages in question. (People who are having trouble – if you enter these directly, does it work?)
https://www.lesswrong.com/allPosts?after=2018-01-01&before=2019-01-01&limit=20&timeframe=monthly&includeShortform=false&reverse=true
https://www.lesswrong.com/allPosts?after=2018-01-01&before=2019-01-01&limit=100&timeframe=allTime
Those both work.