SPFx 1.6 was released recently with new features which support secure connection to Azure AD protected APIs. During upgrade some things in your tenant and Azure AD were changed. For some users migration didn’t go smoothly and they saw strange errors in different places. I’m in the group of users who experienced errors and had to fight with them. Fortunately really smart guys from Microsoft and community resolved everything. There are a few issues on GitHub related to SPFx 1.6 errors. It might be difficult to go through all comments and find an answer on your concrete problem. I see that more and more people struggle with the same types of errors, I sum up potential errors and their resolutions in a separate post.
Basically there are two types of errors you might see when trying SPFx 1.6 features. They are described at GitHub SPFx The user or administrator has not consented to use the application with ID and SPFx suddenly stopped working; experimental feature error. More...