When you make a switchover in Cloud Control 12c, it might show an error message to prevent you to proceed, especially when you are using Internet Explorer as browser. The error message is as below:
Enterprise Manager is unable to proceed. The most likely cause of this is that database session information could not be retrieved or that EM was unable to process a user-invoked bookmark. Press OK to continue.
Unable to Switchover in Cloud Control
I found a document 1512567.1 from MOS that can explain the error sourced from a bug and can be worked around by:
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Solution
Please use another browser than Internet Explorer 9 and Firefox 10.
Wait a minute, if it is caused by compatibility problem of the browser, there is no need to change your Internet Explorer, just set "Compatibility View" in your IE.
Find the path to set compatibility view in IE.
Internet Explorer - Compatibility View
Add the site that you want to have a compatibility view.
Internet Explorer - Compatibility View Setting
Now, you can try to switchover again. I tried, it succeeded.