Render nodes not contributing

Any development on this problem I had a while back?
I get it from time to time. No idea what’s causing it and how to get them back online.
Here’s the relevant text in the progress window:


Starting DR 0
dr host: 10.88.196.103:20211
dr host: 10.88.197.6:20211
dr host: 10.88.198.192:20211
dr host: 10.88.195.194:20211
dr host: 10.88.196.135:20211
dr host: 10.88.198.205:20211
Host 10.88.196.103:20211 added to waiting list
Host 10.88.197.6:20211 added to waiting list
Host 10.88.198.192:20211 added to waiting list
Host 10.88.195.194:20211 added to waiting list
Host 10.88.196.135:20211 added to waiting list
Host 10.88.198.205:20211 added to waiting list
Could not use any of the selected (6) hosts for distributed rendering!
Preparing camera sampler.

Sometimes it happens to me. Restart the license server and also re-start the spawners in the rendering nodes. They should then resume rendering after a short while.