We had the same issue. The comment on pull request #166 provides a fix for the problem.
After setting the settings 'Read Timeout' to 30 and 'Thread Executor Size' to 10 the problem was gone.
Thanks for the suggestion Daniel Daehler
I'll look to give it a go in the morning, when no jobs are running.
I started getting the same error on all builds on the changelog annotation step after upgrading to 3.0.3, I also found Read timeout and Thread Executor Size both set to zero in the config. It seems the defaults and/or migration is not done properly.
Setting these to non-zero values seems to have helped in my case, but such manual action shouldn't be necessary.
I can confirm setting the Read Timeout and Thread Executor Size to non-zero values has remedied the issue.
Fixed in 3.0.5