Can't start DHIS2

My previously working DHIS2 installation has stopped working. The startup works, I see steps 1 through 10 completing OK in the log, but when I try to connect to the login page, I get the a 500 error:

javax.servlet.ServletException: Filter execution threw an exception

with root cause

java.lang.NoClassDefFoundError: org/springframework/web/context/request/async/WebAsynchUtils




Any suggestions? It looks like I have plenty of memory available.