1.14.1 is definitely having a bug. But we faced same so try restarting the machine / using another machine or send the xaml to other developers and see if they are facing issue once degraded. Even we took 2 days to solve trying all but at the end once the issue is gone it worked.
One thing that we tried is we went to .nuget file of other machines and got the system.text.json and copied to our machine. You can try the same.
cheers