Syncing a repo which is a number of commits behind its parent freezes.

We have a few repositories with several forks. We have the forks a few commits behind their parents, therefore we get a message like:

This fork is XXX commits behind XXXXXXX. Sync now.

Clicking on Sync now opens a dialog window to do the merge, which used to work, but now, clicking on Sync, leaves shows a wait indicator (spinner) for a short time, and then returns to the same window, without doing the sync.

This makes it impossible to sync repos using the web interface.