I have already configured the Digital Ocean slave machines in the Jenkins CI and it was working fine. but all of the sudden the slave couldn't created in Jenkins server at last three days, when I updated the Jennkins server and changed the workspace.

When I see the error log, I got these two different errors. How could resolve these errors?

Jenkins Version: 2.90

Cannot create a droplet with a smaller disk (20GB) than the image (320GB).
com.myjeeva.digitalocean.exception.DigitalOceanException: Cannot create a droplet with a smaller disk (20GB) than the image (320GB).
    at com.myjeeva.digitalocean.impl.DigitalOceanClient.evaluateResponse(DigitalOceanClient.java:1210)
    at com.myjeeva.digitalocean.impl.DigitalOceanClient.executeHttpRequest(DigitalOceanClient.java:1155)
    at com.myjeeva.digitalocean.impl.DigitalOceanClient.doPost(DigitalOceanClient.java:1117)
    at com.myjeeva.digitalocean.impl.DigitalOceanClient.perform(DigitalOceanClient.java:1074)
    at com.myjeeva.digitalocean.impl.DigitalOceanClient.createDroplet(DigitalOceanClient.java:296)
    at com.dubture.jenkins.digitalocean.SlaveTemplate.provision(SlaveTemplate.java:224)
    at com.dubture.jenkins.digitalocean.Cloud$1.call(Cloud.java:224)
    at com.dubture.jenkins.digitalocean.Cloud$1.call(Cloud.java:216)
    at jenkins.util.ContextResettingExecutorService$2.call(ContextResettingExecutorService.java:46)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:748)

Unexpected exception encountered while provisioning agent jenkins-slave-slave-7be01428-2fea-451d-bdf8-e7e4dbfc3bc3
java.lang.AssertionError
    at com.dubture.jenkins.digitalocean.SlaveTemplate.provision(SlaveTemplate.java:229)
    at com.dubture.jenkins.digitalocean.Cloud$1.call(Cloud.java:224)
    at com.dubture.jenkins.digitalocean.Cloud$1.call(Cloud.java:216)
    at jenkins.util.ContextResettingExecutorService$2.call(ContextResettingExecutorService.java:46)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:748)