@Carl S ,
Do you mean that you cannot open Skype for Business Server Topology Builder?
If you could log in Topology Builder, please try to follow the steps to change Witness server in SQL mirror configured for Skype for Business Server:
- Open Topology Builder and download topology from existing deployment.
- Go to the properties of the pool for which you want to change the configuration and go to the Association under General > Uncheck the witness server option and publish the topology.
- Once topology is published successfully, go to the shared components and delete the witness store which you have removed in previous step. Publish the topology once again.
- Once topology is published successfully, go to the Primary SQL server and select each Skype for Business database one by one and go to the properties > Select mirroring from left panel and clear the witness configuration and click on OK. Do the same steps for all the databases.
- Once done, go back to the topology and define new witness server and publish the topology.
If it doesn’t work, try to build a new witness with the same configurations.
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