Symptoms

Out of the Box Worklist Error.
We are using the out of the box Worklist control for our project.
Yesterday evening it stopped working in Dev. The list does not load any tasks, the loading loop just spins and spins.
We made several changes to the workflows and forms yesterday and I can't rule out those were the cause - but to be sure we rebooted both of the servers in the environment. Then I removed all of the tasks/instances of each workflow (and associated log items), deleted all the old versions of the workflows and then we whipped away all the data from our application's database. Finally we restarted all the K2 windows services.
After all of that the list still doesn't load.
We have an "Environment Validation" workflow that sends emails and tests the environment. That workflow has a form that also uses the out of the box Worklist. That worklist does not load either.

 

Diagnoses

Reviewing Environment Field for this environment (Development), we noticed that none of the Web Service URL type are set to Default.
 

Resolution

Setting one of the Web Service URL environment as default environment filed resolved the issue.