Try this:

• open hudson's main configuration page
• add a slave with reasonable settings
• save this configuration
• (all is fine)

• open configuration again
• "delete" the slave
• save the config

you'll get an error!

In this case I have to restart my JBoss server to get hudson in a useful state.
No manual editing of config.xml and reloading helps. I found this problem in
1.59 and 1.60.