Recently at a client, the SSIS Server was giving an error message which I had never seen before every time I tried to run a package.
Changing protection level: Success Message: An error occurred in the Microsoft .NET Framework while trying to load assembly id 65537. The server may be running out of resources, or the assembly may not be trusted with PERMISSION_SET = EXTERNAL_ACCESS or UNSAFE. Run the query again, or check documentation to see how to solve the assembly trust issues.
Googling this error led me to some old SQL 2005 and 2008 articles relating to proxy accounts, but that did not seem to apply. Upon digging deeper, the server had been rebuilt and the SSIS database restored.
The proper method for restoring the SSISDB database is located here: https://msdn.microsoft.com/en-us/subscriptions/downloads/hh213291.aspx