There's an article about this on the MSDN blogs.
Click here to read: Error: 26 - Error Locating Server/Instance Specified
Suggestions from the article:
The client stack could not receive SSRP response UDP packet from SQL
Browser. It's easy to isolate the issue. Here are the steps:
1) Make sure your server name is correct, e.g., no typo on the name.
2) Make sure your instance name is correct and there is actually such
an instance on your target machine. [Update: Some application converts
\\ to \.
If you are not sure about your application, please try both
Server\Instance and Server\\Instance
in your connection string]
3) Make sure the server machine is reachable, e.g, DNS can be resolve
correctly, you are able to ping the server (not always true).
4) Make sure SQL Browser service is running on the server.
5) If firewall is enabled on the server, you need to put
sqlbrowser.exe and/or UDP port 1434 into exception.
Based on what you tried already, then maybe it could be a name typo, DNS or try both Server\Instance and Server\\Instance
in your connection string.
The MSDN article have other notes to review as well.