Dear Jenkins community, 

I want to report you that while we upgraded Jenkins 2.497 -> 2.500 and Kubernetes plugin from 4306.vc91e951ea_eb_d to 4324.vfec199a_33512 in config.xml the <clouds/> tag became empty and we lost all our cloud based agents. 

I assume the issue is on Jenkins upgrading because after these steps cloud agents aren't empty: 

1. I reverted Kubernetes plugin from 4324.vfec199a_33512 to 4306.vc91e951ea_eb_d
2. I restored the old config.xml and copy-pasted the <clouds>...</clouds> to current config.
3. Restarted Jenkins.
4. Cloud agents were here.
5. Upgrading manually Kubernetes plugin from 4306.vc91e951ea_eb_d to 4324.vfec199a_33512 from Plugin manager.
6. Restarted Jenkins,
7. Cloud agents are still here.

Because manual upgrading the Kubernetes plugin didn't repeat the issue - I assume the issue is on Jenkins upgrading itself.
Type of the installation - Docker. 

In attachments you can find also plugins list before and after - maybe another plugin affected on it.