Not knowing your prior and existing Tableau Server environment (before and after the upgrade), I cannot say for sure, but below are thoughts and suggestions:
If you login to Tableau server, and are only provisioned on one Site, you will automatically be brought to that Site. If you are provisioned to have access on multiple Sites, you will be asked to choose a specific Site. In addition to the upgrade, it appears other changes were also made. I suggest reaching out to your Tableau Administrator.
It appears you are attempting to use Tableau Desktop to open a workbook where the data source (to feed the workbook) is published on Tableau Server. One guess is that this path information was saved for you, and since the migration, the path has changed. Therefore, you will need to again choose which data source to use for your workbook. If you do not see the data source you used to use, it could be no longer present or you may no longer have the right credentials to access it. Again I suggest reaching out to your Tableau Administrator for an explanation.
I hope this helps.
I would download the workbook from your Tableau Server and once you have checked all the connections, I would republish it.
Just thinking out loud here ....
Try to republish the workbook to the Tableau server 2018. Pay attention to the Data Sources settings (click Edit) and see what Authentication is listed there.