this is what happened:

in build #x, the update failed because I changed credentials:

ERROR: Failed to update https://xxx
org.tmatesoft.svn.core.SVNCancelException: svn: No credential to try. Authentication failed at 

in build #x+1, I updated credentials and the build succeeds. However the Subversion plugin reports it as having no changes:

Fetching 'https://xxx' at -1 into 'D:\.hudson\jobs\xxx'
U         xxx\yyy.cpp
U         xxx\yyy.h
At revision 860
no revision recorded for https://xxx in the previous build

The build overview page also says 'No changes.'

This is not really correct, and it's especially annoying since we also use the Jira plugin: since no changes are seen, it won't update the corresponding issue so we never see this build in Jira.