Before you delete queries, I would check that your dashboard client is on the same patch version as the server. I made this mistake and received this error. Also check that the security token server is running in the CMC. If you segemented it from the APS, check that you set the new service as to start with BOE. I found that we would restart th BOE server, but the STS server was not set to automatic and we would get the connection error especially if connecting with authentication type other than enterprise.
The last thing I would check is that on your dashboard client PC, turn off windows firewall and windows defender. I found this blocked network ports to BOE server.
Good luck!
Lee Lewis