When you open documents posted to the SharePoint site in the GCC High enclave, you will likely be asked to authenticate using enclave credentials. Later, after you disconnect from the enclave, you may see an orange triangle at the top of Excel application window (next to you name).
It is asking you to sign in again, however, you should first attempt to close the application and reopen it. This appears to reset the application back to the standard user and resolving the error.