These two levels in my private courses can’t be opened. The courses are older than a year and I hadn’t edited them before discovering this a week or so ago. Now I added a new empty level. My other bigger courses with 500 or 1000 items per level are fine (like this one). I think these are my only courses that have a level with more than a 1000 items.
http://www.memrise.com/course/504548/300-1300-my-hsk-5-no-typing-pinyin/1/
http://www.memrise.com/course/502606/my-hsk-5/1/
Reviewing works fine.